would your dd try it if she thought it was a good idea?
you could try reading through the Sunderland ARU (goddammit I can never remember what they are called now! - something like ESPA?) stuff, and maybe doing the Sunderland test (it's a urine test and a form to fill in, so nothing intrusive).
then she could quiz the nice people (who aren't at Sunderland anymore, but wherever they are they are really nice and helpful!) and talk it over with them - you get all kinds of graphs etc with the results, and a through explanation of why and how it might be affecting you.
would thathelp get her onside a bit do you think?
there are many ways to cut down gluten/casein without doing full gaps (I don't do gaps; keep thinking about it. dd1 has been gf/cf for over 4 years now, and we don't use a lot of the gf substitutes - bread occasionally, and that's it really)
if it is an issue for her, then just giving it a break for a while might help her to see (on re-introduction) how rotten it makes her feel?
there are a couple of books which might be useful:
this is a really good introduction to the whole topic and
this one has some strategies to help with food issues (and if there isn't a strategy, it does also help to know that other peopl have come through the same stuff
I do sympathise. dh thinks that his dd (my dsd) would benfit from a gf/cf diet, but she refuses to even test for it. as she is an adult now, there is little we can do about it. it must be harder with older children who have 'real' opinions (sounds wrong, but with dd1 I changed her diet when she was 2.6 - she really didn't get much say! dd2 was gf/cf from weaning - again, little say or choice in the matter! dd2 does eat gluten now, but is still dairy free)