Well, cheating is on both people if they both know the other person is married (or in a serious relationship). If Aki didn't know she was married then that one would be on her. But since he does know she is married it's on both of them. I mean I wouldn't fault the guy if a hot woman gives out up to him and he didn't know she was in a committed relationship. But I WILL fault him as well in this instance because he knows.
All that being said, he should take a whack at the friend. If it doesn't work out then it's good material for his manga. But we all know Sensei doesn't want anybody but her to take that v-card ... LOL. Then again that's probably why I have a strike against Sensei ... she's too selfish with HIS D.