Best friend being cheated on

I'm wanting a bit of advice because I feel like I'm stuck and don't know what's better to do.

So there is a girl who I used to be best friends with, now we're not as close, we talk every few weeks but not too much. She recently celebrated 1 year with her boyfriend and from what she told me she is planning her future with him.

Last week i seen an old friend from school and we grabbed coffee, she asked me if I stay in touch with the girl I mentioned above and so I said that we sometimes talk. She has then told me and showed me messages of the girls boyfriend asking to meet up to talk and he confessed that he cheated on her 3 weeks ago and is thinking of breaking up with her. I feel like I should tell her, but if he denies it I will have no proof. Everyone is telling me not to get involved but she was like a sister to me and I feel extremely bad. What would you do?