Should I butt in or not? (Friendship/relationship dilemma)

My best friend recently broke up with her boyfriend who is two years older then us and will be going off to university in the autumn. They supposedly broke up because he was too stressed to concentrate on their relationship. She didn’t want to but she tried to be as understanding as possible and he kept reassuring her that they could get back together after exams are over (which I think is totally unfair). So for the past week that they’ve been broken up they’ve still been texting each other most days and they’ve spoken a little bit at school but she has been having a really hard time because she feels like there are boundaries to what she can say and do around him.

Anyway, before they started going out, he had briefly ‘talked’ to this horrible girl who is also in mine and my best friend’s year who I was once good friends with a long time ago and so was my friend up until her and her ex went public. So this girl and my friends ex and had a few conversations but she was way too clingy and immature about it so he wasn’t interested. When my friend and her ex revealed that they were seeing each other this girl went nuts even though she was being hypocritical because only a few months earlier and she hooked up with a guy who was going out with this girl who the bitch is good friends with - let’s call the bitch Sarah and my friend Jane. So she went nuts and then ended their friendship. Jane’s ex was also in a car crash at the end of last year when they started talking and he was hurt quite badly - Sarah once said ‘I wish he had died in that car crash’ which was just unforgivable.

So the other day I was sat behind Sarah in a theatre on a drama trip and I noticed she had recently been snap chatting Jane’s ex and I got really angry because this would be something she would do and if Jane’s ex was messaging other people already then he’d broken his promise with Jane to not talk or get with anyone else. So I messaged him very calmly and just asked what was going on because I want to look out for Jane; he said he that it was completely innocent and they were just checking that were I hard feelings after what went on in the past. I was a bit suspicious but I didn’t want to tell Jane as I thought it would upset her unnecessarily.

Yesterday, whilst at school, it was revealed that he had cheated on Jane several times in their relationship and had hooked up with people since they broke up. He had also been getting very close with a previous ex who Jane has always felt intimidated by. Jane was livid and when he found out she knew he first started like teasing her and being really insensitive - indirectly and through text. Before she found out he would call her up crying and start driving to her house and then turn around. Just before our last lesson she spoke to him and he denied it all even though she had been told by best friends of these girls that he had hooked up with. It had also been rumoured that he was talking Sarah.

Last night she had been texting him and he eventually admitted to two of them - ambiguously of course.

My dilemma right now is should I tell her that I saw him on Sarah’s phone and overheard her mention his name multiple times in conversation and that I spoke to him or should I just leave it? Because I still feel like she’s not sure about what’s happening and that there’s still a chance she would go back to him so I think this could help steer her in the right direction?

I would really appreciate any help I could get! Xx