This really is about a friend

My friend has been in a very long term relationship she got with her man when she was 14 or 15 they have been married for 20 something years she is always complaining that her man never has sex with her she is contemplating having an affair with a really hot younger man when she asks me what do I think it's so hard for me to say don't do it because do I want my friend to be unhappy & die sexless it is a strange thing cuz other tan the sex they seem perfectly matched I want to say use a condom & have fun but what would you all say