First of all, I hope you have severed all ties with that woman. I mean RUPTURED here!
Second, you HAVE to tell her DH, especially if there is a chance the child may not be his.
If he IS a good dad, good DH etc, and genuinely repentant, then try and give it a go, but all contact with that witch has to stop.
If there is so much as a missed call, then that's it. He changes his email, his mobile and if possible, i think you ought to move house.
Do everything to eradicate her from everything. You have not lost a friend, she was a parasite feeding her own greeds on the host of your marriage. If you get through all this, no need to tell your parents/family etc. It's not their business, but it will be harder on you going through this on your own... but you always have MN!!
History indicates future behaviour, so he needs to be absolutley sure that you will be alert to him and that if he values your family, then he toes the line, if not, hit the road!