Husband is attracted to my adult daughter

My husband has been in my daughter's life since she was 4. We've always had a great marriage but for the last month he hasn't really been wanting to have sex with me. My daughter lives with us and she's always been pretty. But I never would have thought he would be attracted to her. I found out because at first I thought he was cheating and went through his phone and he's telling his brother how "sexy" my daughter is and sending him pictures of my daughter in a bikini from her Instagram. When I spoke to him about it he was super apologetic and said that he knows it's wrong but he can't help it. He said she's temptation. I have been looking for advice and while my friends think I should kick him out my mom thinks I should have my daughter move in with her dad because men are biologically visual creatures. I can't blame him for his attraction and that I should get any type of temptation out of our home. She also pointed at that my daughter is 20 with a job and should have her own place.

I'm stuck on the entire situation. On one hand my mom is right. My daughter is 20. She does have a job and the only reason she isn't living with her dad is because she does not get along with her step mother. It's not like she's 16 and I do believe in trying to save my marriage at all cost. On the other hand my daughter has lived a very traumatic life already. She suffers from BPD which effects her relationships with other and I feel her step mom doesn't get it.

Only looking for advice. Not judgement. It's easy to judge when you haven't been in my situation.