First thing first, the title should already be a red flag. There really should be a whole a manga about this. The answer is a clear NO.
Girrrl, if you claim to be someone's best friend, you better act like one. I can understand you debating whether to tell your best friend about the cheat because you don't want her to feel heartbroken. But the reason for your hesitation is because you realise there is a chance for you to have sex with your friend's boyfriend, because you like him? Nah ah. You messed up.
The way you act is less of a best friend and more of a backstabbing green tea bitch. You might as well call your best friend an acquaintance. I get it, you like her boyfriend. Fine, sometimes people just crushes on people they should not be crushing on. But seriously, have some decent morality at least.
The only real victim here is the best friend. The rest are toxic af.
XY is a horrible person but a great villain.
I do find the XY and XXC ship to be toxic because of how twisted XY show his affection to ppl he actually like. (I'm more of a SL×XXC with XY and A Qing as their children) And ppl have to stop defending XY because he is a horrible person since he, oh I don't know, killed an entire sect because two members of that sect wronged him, killed SL's family and make him blind to take revenge against both SL and XXC because they tried to catch (turn him over?? I can't think of a term here =v=ll|) him.
This is just a very vague option of mine, I actually typed out a more detailed view but I accidentally deleted it.
And I'm tearing up already...