king khan...

Take her in a place or hotel where you can explain... what really you are feeling about her behaviour, and make her realise that she doing is wrong,
if she ask you ,why you wanna screw it up, tell her that i am feeling pain dont know why, may be i dont wanna make my friend to do mistake.. . .. .

But if she is serious about the relationship or is she is happy with him, Then just tell her i am happy to know that you are not going to make any mistake, and come back..let her stay as she wish...