Well she clearly did say "use me" to Don, so it's still not his fault. Don being a doormat back then left himself nothing but to agree.
Didn't she also use Don?
Just because she said it, doesn't mean it was the right thing to do. She was clearly emotional unstable. He was maybe her first love and that's why she wouldn't let go.( although I personally think the character had in generally not enough depth, which made it kind of unrealistic that she wanted to be used by him) I know it was wrong from her that she still wanted to be on his side, but instead to leave her he continued the relationship. He only thought about himself and not about her.
But she just thought about herself too when she offered herself to Don people never do anything out ofpure generosity we always seek some sort of recompense whether this may be emotional or material is another thing.
So she saw an opportunity to gain Don's heart since he was in a very emotional state He should have rejected her but he didn't however he never said she would love her back because of that. The reason why he feels as if he owes her something is because he has seen her efforts and dedication not because he promised her anything. I always have problem when people somehow never want I woman to fully suffer the consequences of her actions it always becomes "because someone did this and that poor her she doesn't deserve it" She and don had the same age out of them both I will say Don was the one tricked since he was the one going for real hard situations not her.
THIS
I actually only feel bad for nami lol she is the only one that really wasted her youth in my opinion. I know she wanted that but to use her like that is just disgusting he should have told her no and nothing else. I hope he leaves her