best friend...

She got mad because she didn't think her boyfriend would propose to her so she went out and cheated on him not just once and not with just one guy.. but today..he messaged me on Facebook to have me take her somewhere so he can propose..
I want to tell him but it's not my place.. but it hurts because this guy HE IS ONE AND A MILLION he works two jobs so she can just stay home and do whatever she sonly 22 and has no interest in college or gettinga job. He deleted alll social media for her. He has. Ever even touched or looked at another woman. He even changed his job just last year because he worked with a girl she DID NOT like and I have sat and watched her tear this man apart..