I agree with the above. Narcissists tends to be nice and helpful to everyone else. Right now, you are everyone else. Your not within his family system. Your an outsider so he’s being nice and helpful to you and yeah he’s definitely sniffing around and I bet OW is upset (good she deserves it) but he doesn’t deserve trust from you. Just use him for whatever you can (dog walking, baby sitting) but don’t trust him
