My bestfriend (23f) cheated on her boyfriend (22m). Should I tell him?

This girl is my (f23) best friend since we were kids. I’ve spent 10+ years being her friend but I can tell you she’s not a nice person. She has never been true to anyone in her life.

She cheated on her wonderful boyfriend with her toxic ex while they were living together and were committed to each other. She told me blushing and asked me to keep it a secret. It’s been 2 years since I’ve known and it eats me to hide it from that genuinely nice man.

She doesn’t even give emotional or mental support to her boyfriend and is a financial burden on him, as he pays for every single one of her expenses including the rent and bills.

I’m not that close to the guy but I genuinely feel sorry for him and wanna tell him but this means the end of my friendship with my best friend. What should I do?