Picture this:
Your best friend, whom you've known for however many years it took for him to climb to the rank of "best friend" has a girlfriend (although it's irrelevant to the topic, she is pretty but not amazing, and for the record you are not attracted to her at all). They've been dating for a while, let's say atleast a year, and they've had no prior problems. Said best friend's girlfriend starts hitting on you, flirting with you constantly, etc.. Your best friend is oblivious to this, and very much into this girl, so much it would devastate him if he would find out that she's even flirting with someone else.
What do you do? Do you let him know his girlfriend is flirting with you? Do you not tell him for the sake of their relationship? Do you approach her and tell her to back the fuck off? Do you threaten her that you will tell your best friend if this keeps up? Do you ignore her and risk her flirting even more (because we all know women want what they can't have)?
You know the saying "bros before ho's" and you live by it, but for the sake of your best friend's feelings, you opt not to tell him. Eventually she ends up cheating on him, and they break up. Would you tell him that she flirted with you, or would you keep quiet about it? If you did tell him, when would you tell him? As soon as they broke up, or once he was over her?
If you did tell him, him and his girlfriend would get in a fight and end things and he would blame you. Would you risk your friendship with him or risk his happiness if his girlfriend decides to cheat on him completely?
This is a hypothetical situation. I'm not and hope I never find myself in this situation, it was just something I was wondering about.