Should I tell her??

So I have this ex that I’ve been with for over 8 months (he literally ignored me for a week an I just let it kinda happen) one day he randomly texted me an asked me what was wrong with me an I told him I wasn’t going to beg for his attention, he got all mad at me an told me he was done with my bs an how I always bitched at him an he was tired of dealing with my shit, An I did exactly what I told him An I didn’t beg for him. The whole time we was together I didn’t like him being around his ex, one bc while we was together he told her they were meant to be more than friends an it wasn’t supposed to be like it is two they have a vv long past that’s not really my place to explain an the week he told me he was done he randomly started talking to her again. I was crushed an felt so stupid, she ended up telling him that she just wanted to be friends with benefits basically an he decided to text me an tell me he missed me. Ik right the audacity? Anyways I told him I wanted ya to be friends but it hurt me to much when he left the first time so I couldn’t be in a relationship with him, An he pretty much told me if he couldn’t have me no one could. I was pretty okay with this at first bc I didn’t really want to be with anyone at this point anyways I was still too hurt an I missed him. He got with a girl that could be his little sister, An I was crushed don’t get me wrong but I felt awful for this girl she lost her brother like two years ago an she didn’t know how he is. A week of them talking he came in to school with a hickey the size of a baseball on his neck (I’m a sophomore in collage)an the girl heard about it an she cried an I feel awful for her, she still doesn’t know who gave him the hickey ( it was his friend with benefits) but the thing is the girl doesn’t know abt his friend with benefits an I don’t want this girl to get hurt worse than she already is, should I tell her who gave it to him or just keep my nose out of it bc it’s not my business An me An this girl aren’t really even friends?