Beer, however, DOESN'T AFFECT HER AT ALL. Further research indicates that this is becausue the traces of gluten in the final beer product are absolutely tiny.
Get ur mate to try a normal, commercial beer, he'll be able to tell if there's enough gluten in there to affect him, don't be afraid of it. Dunno about homebrew, I wouldn't be so confident, but if he just tries one homebrew, the worst that could happen is he'd get a bit of a sore tummy and stuff. Tell ur mate to give it a shot, the gluten in beer is VERY minute.