So he met her for work, so has he been in contact with her the whole time?
What is he telling you about that?
I am sorry for your financial and health situation, but until you meet with an attorney, you really don't know that you are stuck. There are laws, and rules, and rulings in place to protect kids, and spouses, especially those with disability.
Start stashing some funds as well. Take an extra 10-20 out every time you go to the store. This adds up. Gives you a nest egg.
Do you know what you want? Would you stay if finances weren't a consideration? Is he being remorseful? Did he do anything to change himself when he cheated 4 years ago? (I don't really expect you to answer these here, but they are things you should try to look at as an outsider, so that you can gain some perspective).