My 19 yr old has been talking to a younger married friend of ours

The wife called me in shambles a few days ago because she needed someone to talk to about that fact that she thought her husband might be cheating on her.  We had just come back from vacation and it was very sudden. I had no clue that my 19 yr old daughter would have anything to do with that until today. They haven't done anything so far physically that I know of but talk on the phone which is bad enough as far as they've told us.  The husband came over to my house earlier today to ask my husband if it was okay for him to pick our daughter up from the airport because he is interested in her and that is how we found out she was the person of "interest". The nerve of him, what did he think we'd say. He isn't the ideal person for our daughter or anyone else. He's married with three young children. He must be out of his mind!  After talking to my daughter about the significance of wrecking a marriage and breaking up a family, she decided that she will back away from this guy and not pursue him.  The married guy said the same thing. Somehow it was all to easy and now I'm really worried.  My husband told the marreid guy we all can no longer be friends in order to avoid the awkwardness of everything.  The wife still does not know who the person of interest was and I sure don't want to tell her.  I feel horrible that my daughter would have anything to do with some of the pain she is feeling.  I hope she doesn't call me anytime soon because I am stumped...