Is she on any medication for the MG?
Please be aware, if she has an infection of some sort (swelling of lymphs), some medicine can worsen the MG. So if you're primary doc prescribes something for the infection, please look it up and/or ask your neurologist.

Or do you mean the thymusgland is swollen on the MRI? Because that is a "normal" thing for MG patients. This could be a thymoma, or thymus hyperplasia. It's also possible it doesn't really have to do much with the MG, because she's so young: it shrinks with age, the first years of your life it's much bigger.

Anyway, I just wanted to add this; I don't know if she's already menstruating, but be aware that it sometimes gives more weakness (hormones) that week or the week before. Especially on young woman, because it just tends to be not that stable anyway (menstruation I mean). Just something to keep in mind.

I wish you all the luck, I also got sick when I was a young girl.