Parkfell
While the condition progresses slowly, an attack can progress quite quickly, certainly in 24-48 hours. Since attacks can affect eyesight as well as any motor nerve, there are significant implications for professional pilots.
Scandi
The difficulty with MS is that at diagnosis (subject to BC's comments) there is no indication of the rate of progression of the MS. I've seen people struck down by it quite quickly and in a wheelchair within a couple of years, and there are others whose progression is very slow.
My wife was diagnosed 20 years ago. Her first symptoms were visual disturbances, which are common, but not often followed up. Only when she had problems playing tennis did she go to a physician. She has since had three children, but does have problems with stamina, and has a limp following attacks to the R side of her body. She certainly would not be able to hold down the job of LH pilot even if she could pass the class 1 with restrictions.
At this stage your friend will be feeling very worried about his future, so do what you can to support him. It's very tough.