Hi
Guy who organises our training runs (Early 50's) was diagnosed about 4 years ago with MS. He was a very keen cyclist, doing LEJOG's solo in 5 days and the 1200km Paris-Brest-Parsi, so a real keeny, however he had to give-up riding about 2 years back, though I'm not sure it was balance related necessarily, but he'd lost the will as he found it very hard to have any kind of useful (in his terms) form. He still rides a motorbike though. His other passion was the Euphonium but he is finding that hard now too.
I'm afraid this is not heartening, but is the only case of a cyclist with MS that I know. The condition may vary in it's rate of onset with different people and my friend may be unlucky, but it is sad to see how he has deteriorated (he still organises the club training and does the timing and handicaps). He has great strength of character and is a weekly inspiration.
I guess what I'm thinking is that it's hard to predict the pattern of events for your friend, but she may find that she takes to cycling like a duck to water and then find it difficult in a short time period to continue. I wish I could be more cheery