I've been wanting to get some advice on this for a long time. My friend [Brad] who I've been friends with for 15 years, is cheating on his girlfriend [Kate] of 6 years. I'm not that close with her but I do see her often and we have a good time when we hang out. She's such a smart, nice and wonderful girl. I know Brad is cheating on her because he tells our group of friends all the time.  He meets girls off of tinder at hotels, brags about unprotected sex and even had sex with one of my other mutual friends who I used to work with (I could not believe it). When his girlfriend is out of town he invites his ex over to hook up with her and he has also admitted this to me.   
I've asked my other friends if I should go ahead and tell her and everyone has been telling me (1) not to get involved and (2) that she probably already knows (3) she might even tell me to take a hike and then I will end up being an outcast from the group, lastly (4) I was Brad's friend *first* so my "loyalty" is to him. I thought about writing her an anonymous letter but I have no idea how I would do it. I'm so disgusted by Brad that I have stopped hanging out with our friend group if he is there. I can't even look at him.  
Whenever he told me about any rendezvous he had I would seriously scold him and tell him that what he's doing is fucked up and he would always say, "I know, I know, I need to stop."     
So the reason why I'm asking Reddit for help now is because 
another friend of mine told me yesterday that he is planning on popping the question to Kate and asking her to marry him. I feel the need to tell her even more now. Any advice on how I can do this? I really don't want to hear that I'm a bad person for not telling her so far. I already feel horrible and I'm probably the only one in our friend group who even gives a shit.
user
Friend is cheating on his girlfriend and I want to tell her that he's
assistant
I've been wanting to get some advice on this for a long time. My friend [Brad] who I've been friends with for 15 years, is cheating on his girlfriend [Kate] of 6 years. I'm not that close with her but I do see her often and we have a good time when we hang out. She's such a smart, nice and wonderful girl. I know Brad is cheating on her because he tells our group of friends all the time.  He meets girls off of tinder at hotels, brags about unprotected sex and even had sex with one of my other mutual friends who I used to work with (I could not believe it). When his girlfriend is out of town he invites his ex over to hook up with her and he has also admitted this to me.   
I've asked my other friends if I should go ahead and tell her and everyone has been telling me (1) not to get involved and (2) that she probably already knows (3) she might even tell me to take a hike and then I will end up being an outcast from the group, lastly (4) I was Brad's friend *first* so my "loyalty" is to him. I thought about writing her an anonymous letter but I have no idea how I would do it. I'm so disgusted by Brad that I have stopped hanging out with our friend group if he is there. I can't even look at him.  
Whenever he told me about any rendezvous he had I would seriously scold him and tell him that what he's doing is fucked up and he would always say, "I know, I know, I need to stop."     
So the reason why I'm asking Reddit for help now is because 
another friend of mine told me yesterday that he is planning on popping the question to Kate and asking her to marry him. I feel the need to tell her even more now. Any advice on how I can do this? I really don't want to hear that I'm a bad person for not telling her so far. I already feel horrible and I'm probably the only one in our friend group who even gives a shit.
user
My friend is cheating on his girlfriend and wants to pop the question to
assistant