As Squirmy said I was also over 40 years before I was diagnosed. Like your daughter I was covered in DH lesions as a child and it was horrible. the doctors told my Mom that I had poison ivy in my bloodstream. Yea right.
Be careful of Dapsone and if the doctor does prescribe it make sure they have done liver panels before and at least once a month during treatment as it can be very toxic to the liver.
I got lucky and within a couple weeks gluten free I stopped getting new lesions unless I got glutened. It took quite a while for the lesions I had to heal and it did take a long time for the antibodies to leave my skin. That meant that when I got glutened I would breakout quickly. Now I am about 13 years gluten free and DH is last thing thing to show up and even then I only get a spot or two. I hope your daughter heals quickly like I did but for some resolution of the lesions can take longer.