I've got MG myself and be thankful she isn't on prednisone. However, if her symptoms are generalized, then yeah, she'll probably need a fast acting drug like it.
I for one found megadosing B-complexes, bovine colustrum, and whey protein shakes (the latter two supposedly for the increased immunoglobulin) helped immensely in the management. However, I did go through traditional treatment and everything else I ever took was mostly to supplement the side effects of the prednisone until I got switched over to Imuran.
There really are no supplements to really assist along the way that much aside from sleep, a very clean, very healthy diet, and the like. Green tea might be a good option and well, if she starts experiencing weight gain because she's tired all the time then you might want to look into some cAMP/cissus.
Me, I'm still waiting on my thymectomy from 3 years ago to kick in.