>A 22 year old woman who is in a position of power is currently pressuring a 17 year old boy who has no interest in her into having sex
Consider, as much as you may dislike Issei, if this is the crowd he regularly has to deal with, I'd say what he did was pretty damn smart, if not responsible.
Consider, he's socially obligated to attend this party. He knows she wants to make a move on him, and judging by what we've seen of her character she might just want him for his money, and so he wants to put her down lightly. Also, he doesn't want to be left alone with her considering what she could do. So, what's the best solution? Bring someone you trust (and if they're a woman that will give the hint that he's not into her-and if she doesn't get that the amount of embarrassment will escalate) to also attend the party. Sure, he has his bodyguards, but there's still no one you can trust more than someone who is literally indebted to you. He doesn't know what she'll do to him so having someone he knows is a kickass Yankee with the ability to fend for herself would be good insurance, and because she's socially disconnected from their clique/upper-class circle, she doesn't have to fear the consequences of potentially pissing off the wrong people because the odds of them somehow even further ruining her life would be lessened.
Issei definitely has done some questionable things, but so far he's always, even if begrudgingly, ultimately done the right thing. I think this is another instance of that because he's not just putting someone in their place for his own satisfaction, but is removing a potential cause of future conflict. Given he's trying to disconnect from a person who is attempting to Weinstein him, I think it's safe to say that what he's doing to get there isn't as bad as it would initially seem. Remember, we're grading on a curve her, and if everyone is an unquestionable asshole and he's only a massive asshole, then he's still better than the rest of the crew.