<human>: In December my best friend's boyfriend died. She's been getting better and I'm really proud of her! Anyway, our little group kicked out his best friend and after she had a long thinking period, decided that his best friend did nothing wrong and started hanging out with him again. I'm also proud of her about that too, but here is where things get crazy.
The best friend of said dead boyfriend is kind of, sexual??? He tried to work the moves on my friend and I (at the same time and is still trying, mind you) and my friend is like "yeah!" So she did some stuff with him and they both decided that she and him would be friends with benefits. Already sounds kind of bad, right? Well, the guy who is now her fwb has a girlfriend who really loves him and I'm lead to believe her "loves" her too.
My friend and I have been friends for 6 years and we've grown an understanding that if one of us is fucking up, then we gotta tell them! I'm always the one fucking up, but right now I feel like she is the one fucking up.
Not only is that your dead boyfriend's best friend, but he has a girlfriend! Like, I'm totally okay with sleeping with dudes I guess, but this isn't good. I don't know how to tell her that I think she is making a wrong decision and that she shouldn't do it, without making it seem like I'm jealous or trying to tell her what to do, because I'm not! I tried to say "Well, what if the girlfriend finds out?" and she said "Only you, him, and I know so how would she?". I tried saying "Well, don't you think that's dirty on your dead boyfriend's part?" but she said "He is the realest guy I'm going to meet in a while, so why not? I'm not going to be a prude and just wait until I meet a guy!" 
I just want her to be safe and I don't want her to have more drama put on her if the girlfriend does find out.

Can you condense the text into a few key points?
<bot>: My best friend is going to start having sex with her dead boyfriend's best friend who already has a girlfriend, and I don't know how to tell her that she is probably making the wrong decision.