Think you need to ask the girl to honestly tell you where she sees this r-ship going, whether it can turn into something meaningful (which probably requires a divorce from his wife first) or are they heading for a painful wake-up call. She might be in denial, enjoying the moment, without considering the consequences of her actions.
As for the guy, sounds like he has to work through issues in his own marriage. It is easy to say "I'm in love" and talk about the wonderful things another person has brought into your life. But if you already are married, this is not only wrong and irresponsible, it is usually an easy way out of dealing with marital issues, such as the couple not communicating like before, the passion not being kept alive (yes, you must work at maintaining the passion in a marriage) or simply allowing too much routine to creep into their lives so that everything becomes boring.
You should ask the guy too where he thinks this can go, and whether he is willing to sacrifice his family and years of his life being invested into it, or whether he should recognise this temporary pleasure for what it is and make a clean break to rebuild his marriage and his family.