Someone is gonna get it

I don't think I've been this upset before. Some guy had the nerve to lie to my best friend about who he was after she gave him her complete trust. He hasn't talked to her in three months and she didn't doubt him. She doesn't know yet though. I don't know how to tell her. Can anyone give me some advice? I don't want her to be hurt but I know she's going to be. I feel so bad. I'm going to fuck him up. pardon my French … Ugh. I need to go vent. He makes me upset And the worst thing is that I can't do anything because I don't know if he lied about his name too.. xc