The mag by mouth is instant diarrhea for some people, so she needs to know that. If she is one, transdermal magnesium chloride ("magnesium oil") can get around it.
I send anyone with such issues to gutsense.org. And if she's on some kind of fiber, like metamucil, she'd be better off without it, IMO.
Another excellent book is "Breaking the Vicious Cycle" by Elaine Gottschall. This has helped multitudes of people with extreme diseases like Crohns, Ulcerative Colitis, and IBS. Fixed 50 years of hay fever for me two years ago.