What do you think about me telling my managers girlfriend that he was cheating on her
Why would you tell her that?
She stole my managers wallet and now I'm worried that he might break up with her and then I will be blamed for the loss.
It's reasonable to do that, cheating on someone is wrong. People should always know when they've been cheated on.
Man, I don't want to deal with this shit.
It's definitely a hard situation to be in, but if you were being cheated on would you want to know. While it's not your responsibility to tell her, it is the kind thing to do.