Well, to answer your question, a marriage is over when one or both parties has had enough and can see life without the other person as a bonus and a good place to be.
I think though, that your friend needs to do everything in her power to save her marriage, whether it be councelling, talking to her husband about her feelings etc, before she throws in the towel.
She may also want to think how she would cope with a sick child on her own. It is very hard being a parent when your children are in good health, so to end up being the majority carer for a sick child will be very, very hard.