Regardless of the state of his marriage, he chose to cheat. This does give you information about his morals, self-control and ability to commit. Still, people do make mistakes and regret them.
It doesn't sound like this was a one-night stand, but an actual affair. So, did his wife 'find out' or did he confess? I would be far more concerned if he was cheating behind her back and was caught b/c that makes him a liar as well as a cheat.
Second thoughts can generally be amended with judicious action; injudicious actions can seldom be recovered with second thoughts.
--Cyteen by C.J.Cherryh