I have not heard of this and I've done a lot of research on MSM. It is a sulfur donor to the body, but I'm not sure how that would explain the weight gain. Is there anything else she has added recently within the past month or 2? Sometimes it takes a while for side effects to appear, at least that is always the case for me.
It is interesting that she looks slimmer, but actually gained weight. Maybe she is just building more muscle in her workouts? MSM is supposed to help with this type of thing.