Exploring the words ‘dinner’ ‘diner’ and dine’ — How are they related?
I teach a bunch of different on-line SWI courses. (See descriptions here.) One of the richest part of these courses are the informal 'after class discussions' and email correspondences between sessions that inform the content of each course.  I have many email correspondences that I mean to share here, and this is just one that I thought I'd share. Hopefully more soon! I've just done some slight editing for clarity and context....
I received a rich message from one of your fellow orthographers in our session that I want to share as something people may find useful to consider. We may not have time to discuss during our official time tomorrow, but it is something we could definitely discuss in the ‘after class time’. 
Here is the message I received...
"I have spent time this week exploring the word ‘dinner’ (and dine, dining etc) with my five year old."
[Note: This comment came from a participant in the SWI for Early Literacy I'm co-teaching with Carolee Fucigna. This set of words that came up in response to a 'What's in the Family?' of activity we modelled. See HERE for a great post/video from Fiona Hamilton (WordTorque) or HERE for a post on this topic by Lyn Anderson (Beyond the Word) and HERE from Rebecca Lovless (Rebeccaloveless.com).]
I provided a fairly long response that may seem a bit crazy! What I mean by that is that some might read this and thing the detail I describe about these words is way too much for a 5-year-old. 
Before that idea becomes a distraction, please note that what I am sharing is not actually targeting a 5-year-old’s understanding. Instead I am targeting information that would help teachersof 5-year-old students be able to make informed choices about how to respond to questions about these three words.
So, please understand that my response below is targeting teacher knowledge. We can discuss how this understanding could inform how I would discuss these words with a 5-year-old and, and what of this information I might specifically mention.
This question parallels another one that comes up a lot in young classes. If we were doing a ‘what’s in the family?’ game with the word ‘play’ we might encounter these words: playful, player, replay, display, playhouse.
After reading the description below, you might want to think about how you would address a child who asks you, “how is ‘display’ connected in meaning to of ‘play’?"
Leave that question for a moment, and consider the text of my response to the the words ‘dinner’ ‘dine’ and ‘diner'….
Ane you’ve hit a family that provokes interesting questions
If we look at the etymology of the words “dinner” and “dine” and “diner” in Etymonline we can see that they all share the same historical root.
Looking up "diner" takes us to "dine" where we find this historical root in Etymonline...
"...from Vulgar Latin *disjejunare, from dis-"undo, do the opposite of" (see dis-) + Late Latin jejunare"to fast," from Latin iejunus"fasting, hungry, not partaking of food
The entry for ‘dinner’ takes us to this...
"...from dis-"undo, do the opposite of" (see dis-) + Late Latin jejunare"to fast," from Latin iejunus"fasting, hungry, not partaking of food" (see jejune)."
So all three of these are related to each other in meaning “to end not eating” from the same "ancestor". So these words are all in the same "extended family" — they share the same metaphorical "great grandma" back in Roman times. These words pass the "meaning test" of sharing the same history from which that meaning originates.
But are they in the same morphological family? They share the same historical root, but do they share the same present day English base? Are they in the same metaphorical "immediate family".
For that we need to do the "structure test" with word sums. And here is where it gets interesting…
We can take the word "dine" as a base, and add an <-er> suffix to get the spelling <diner>
dine/ + er —> diner
Here we see the reliable convention that vowel suffixes replace final, non-syllabic <e>s.
But a word sum with this base cannot result in the spelling <dinner>!
As we see from the above word sum, the <-er> suffix replaces that final, non-syllabic <e>. That suffix can’t replace an <eand double a consonant!
One possibility is that the word "dinner" is a base all on it’s own. Another possibility is that "dinner" could have a base spelled <din>.  But be carful -- if that is the case, this would NOT be the base for the idea of ‘noise’ as that has a totally different history and meaning...
din (n.)
"loud noise of some duration, a resonant sound long continued," Old English dyne(n.), related to dynian(v.)
So for now, I would consider the words “dine” and “diner” to be in the same morphological family (metaphorical immediate family) because they share both the spelling of the base <dine> and the historical root.
However, “dinner” is in the same etymological family (same root — ‘extended family’) with the others — but NOT in the same morphological family because we can’t make a word sum that links the spelling <dinner> with a base spelled <dine>.
So it’s like “dinner” is a cousin of “dine” and “diner” (same ‘grandma’ back in Rome) but “dine” and “diner” are siblings (same ‘grandma’ back in Rome — and they share the same present day family with the base <dine>).
Now all that information is likely more detail than many would  choose to go into with a 5-year-old! However, I would like to use this story to highlight orthographic knowledge a teacher can have to be able to address the spelling-meaning relationships between these words.
Note that I am not saying what educators should or should not discuss with a particular 5-year-old. But if a teacher is working with young children looking for words in a word family of the base <dine>, they are likely to run into words like "diner" and "dinner" and feel stumped about how to explain why one has <n> and the other <nn>. Of course it is fine for a teacher to say, "Whoa! I'm not sure what's going on there." and leave it as a question for further study for themselves. (But the key is to NOT describe this set of words as some sort of evidence of "irregular" spelling.) However, if the information above is understood by a teacher of young students, they can make choices about how to address the structure-meaning of these words. 
The prompt above with the list of words that might come up when investigating words related to the word "play" is a bit different because one of those words is NOT in the morphological family of the base <play> because it is not from the same Old English root that goes back to the idea of  "frolic, perform". That is a bit easier to address because the meaning is distinguished by the different roots. With "dinner" and "diner" going back to the same historical root, we see that it is just morphological analysis that distinguishes them. 
I'll leave the reader to explore the relationships between those words. 
Cheers for now!