I feel sadness for your friend's parents.

Divorce is never an easy thing to do.
However, let us consider the reality of the situation. Your friend's mother has been grievously hurt. She feels humiliated. And, justifiably so. She clearly feels that the trust on which her marriage was based has been violated (which it has) and she feels that she cannot re-establish that trust.
Your friend's father is clearly unhappy being married to his wife, given that he was so cavalier as to carelessly sleep around on her (and having unprotected sex outside the marriage shows his wanton disregard for her well-being).
There are worse things than divorce. One example would be being in a loveless, trustless marriage.
Let's wish them well as they start their lives anew.
(Incidentally, I, personally know of a man who divorced his wife of
50 years. Both are happier, now. And, isn't that really all that matters?)