"Officially"--the answer is no, but there is no reason why they couldn't.
For example--I have no real dietary issues. No allergies--no intolerances. Nothing I didn't choose on my own anyway.
Well with my first pregancy--I couldn't tolerate Italian food.
So I'm 7 months pregnant and my family wants to go to MK and DH and I tag along. Well they decide they want to eat at Tony's Town Square--every Italian inch of it. So when I booked, they actually were able to help me--the sole non-Italian eating person.
The chef ended up coming to my table since they listed it as a food intolerance and I had my pick of whatever they could get their hands on. I ended up getting a Tuna Fish sandwich. I think it came from the Plaza Restaurant (well--at least the ingredients).
So Disney will accomodate. Note--I wouldn't have died to eat Italian food or anything, but they were no less accomodating to me than they would have been to someone with a very life altering allergy.
You might be able to call ahead and arrange so that you friend will not have issues ordering off of the kid's menu.
The reason they don't want folks ordering off of the kids menu's is they don't want them cheaping out. But when you have someone in your party extremely finicky, I don't think the restaurant should give you a problem in trying to accomodate your friend who has a less of an appetite and prefers more "american" fare.
To be fair, they may charge you a little bit more than a kid's meal, since it is an adult--but if you were to just explain ahead of time what the issue is, they should be able to help you.