Aw, that's so sad. The poor girl.
What I would probably do, if I were you, is try to stay out of it as much as possible while still holding up your end of the 'bargain' - be available to see your client during these 3 days, which he would be able to do if he handles his family in a certain way, and then perhaps just wait and see for the long term. He'll probably stay in touch, or he might just vanish, but also be prepared that his family and marriage might completely break down and he might do something a bit desperate and weird like tell you he loves you and wants to move in with you or marry you in Bermuda. Perhaps prepare and rehearse a speech gently suggesting that while you care for him very much as a client, it would be better if he visited a counsellor or therapist instead of trying to make you his girlfriend because he totally effed up with his existing family.
I absolutely hate the part of the job that involves seeing married clients whose wives would be distraught if they knew. One guy spent ages telling me how he was horny and frustrated because he couldn't fuck his wife because she'd just had their second baby. I now have a "please don't tell me about your wife" policy firmly in place!