My friend is upset with her husband

Okay, ladies I think I can see where she is coming from. So her man and her were having some issues back in 2017. And in 2010 her man had a sexual with relationship with some woman, but didn’t met my friend till 2014. So anyways. The woman re added her man on Facebook and he accepted it back in 2017. ( so they have been friends for almost a year now) Now I know some of you might say it shouldn’t matter right.’? But this is how she explained to me to help me understand where she is coming from. She gets that he has a past and obviously that is okay. She gets that, but the fact that they had a sexual relationship he shouldn’t have accepted. No they did not talk, but she was liking his pictures and all even making comments even though he never said it even liked her stuff back and never messaged. She doesn’t think it was fair of him to accept the request a previous fling. I wouldn’t cross the line of re adding or accepting my old fling/sexual relationships.