firstly....he should had not gave her permission to sleep over his place.....she should be really ashamed of sleeping with her own boss, that is plain old disgusting.....this is gonna sound bitter but he isn't faithful towards you because if he was, he would never let the other lady come into his life.....he could had made excuses that he is not looking for relationships but wants to work.
just as everybody is saying....leave him....you will find somebody.....he is a cheater, i do agree with everybody but according to me, he is a player.....he might dump her when he finds somebody else.....there are some girls out there who know that a guy is taken but they still make their moves on the guy who is already taken......whether in a pre marital relationship or in a marital relationship.
you will find somebody who would care for you and be very faithful towards you.
best of luck !!!
it will be harsh but you have to leave him for your own good.....the best thing to do is tell him how deeply hurt you are, it will make him realize how badly he had hurted your feelings then break up with him.