My(M26) gf (F23)sister's(F31)boyfriend(M31) watches photos of her friends in bikini and masturbate

I don't know how should I react on this please help guys. My gf sister's boyfriend literally took pictures of her friends in bikini when they went to goa beaches and kept it in his secret gallery vault. When his gf caught him while we all 4 were having dinner at a mall she slapped him. So I just stepped in between them and took them all out of the mall. When I asked him he just gave me an answer which shook me from soul that he just uses those photo to masturbate instead of using porn as a source. At that moment I just my gf sister that should I ask him to leave or they will talk about it so she asked me to make him leave and I did so. Now I don't know how should we proceed with the situation.