I know how you feel, when my wife left me it was divorce straight off the bat, no separation. Anyway, has your wife given any thought as to who is going to look after her when the MS gets so bad she can't look after herself, it's not as if her parents are going to be around forever.
A former colleague where I work had the same thing happen to him, only she left him for another man, she also had MS. Question is, is her new guy going to hang around to look after her when the honeymoon is over and he has to look after her full time.