Cheating bf with 20+ women

My best friends bf keeps cheating on her!!! This time it was with 3 women and last time it was 5 women. Over the past 4 years they've been together he has cheated every few months. She never leaves him tho and it makes me angry at her and then we fight about it. Idk maybe it's none of my business but I just feel and know she would do better. She left her life in another state to be with him in NC. Then he keeps cheating on her even when she does everything for him even does anal sex now just to please him. What's "funny" is she says the good he does outweighs the bad...bad? Cheating with that many people isn't bad it's horrible and unforgettable. He even cheated on her on there anniversary but she still stays with him. My husband told me I shouldn't be friends with someone so weak and she's really starting to annoying me with this crap. What advice would you give her and me? Any help would be great! Thanks!