Extra marrital love. Do. please guide?

My best friend aged around 45 years ,married and has son who is 22 yrs.now she is in deep love with her colleague who is 29 years old. the man himself approached her and told that he loves her very much.she told him that there is big gap of age between them and she is married. But he told that age doesn't matter.he is attracted by her official as well as personal behavior. She was not happy in married life. Her husband was not up to her expectation ,means,there is no compatibility between them .he takes care of her and does all the duties of a prompt husband but as a couple there is no love ,romance.there are lots of differences in their nature. But her colleague is just like her dreamboy. She found all the qualities in him which she expected from her husband. So they became very close and also had physical relationship.but now that man wants to settle in his life .he is really a good human being .he loves her now also.this is the problem my friend doesn't know what to do.please guide