A cheater will always be a cheater. And if the other woman thinks she's got him and he is faithful to her, she is delusional. I have his messages and videos that he sends me to prove it. He is still cheating on her.
But you know it's okay if he doesn't want Facebook to know about you, because that would never mean he's like ashamed of you or anything. It probably means he's cheating, doesn't that make you feel better?