I bumped into her on a date with a man
Did you really just "bump into her"?
No, I knew that she was going to be there because her roommate told me and I wanted to find out about this guy.
It must be difficult because I know that you are curious, but it is wrong to spy on people as it is an invasion of their privacy.
I think that she is hurting herself by dating a man and I am going to tell her all about it.
It sounds like you care about her a lot, but she should be the one to decide who she dates as it is her life. Maybe you should let her be and find someone who is interested in having a relationship with you instead.