This is just my own "OPINION"
Please no hate
I believe that tsurugi is also partly at fault, so is the girl. If ever that the girl is really his bff's exgirlfriend, Tsurugi should still not do it with the girl without his bff knowing.
If ever he want or forced to do it with his bff's exgf, tsurugi should still ask or consult his bff first. It is not about being conservative or whatever, it is called respect.
Well, there are persons who are okay with that and there are who are not.
I have a real life example:
My bff has an exboyfriend. Her cousin, who is close to her, dates her exbf without telling my bff, so she got mad. She didn't get mad because her cousin is dating her ex, she got mad because her cousin is dating her ex without her knowing.
She is actually fine with her cousin and exbf to date as long as they tell her.
My bestfriend is not conservative or whatsover, she just have different belief from others.
Also, Here in our country there is this rule between friends called "walang talo-talo" which means they should not date with their friend's ex.
But there are some group of friends that are okay with their friend dating their ex.
It is not that I hate Tsurugi, I am just saying that he is partly at fault. I also pity him because he is the only one who is blamed when in fact, the girl should also be blamed.