Pretty certain she only says that to Ivy when she turns him down.
I'd say it comes up earlier and by default:
Python:
r "You're the best, have I told you already?"
l "Yeah, you did."
r "You need to let me take you out for drinks to thank you for all the great work you do."
"It wasn't the first time he proposed that. I hadn't taken him up on his offer yet..."
l "Let's survive this week, first."
If she keeps saying no then the guy should just take a hint already. It also shows she isn't interested in him. Whether this is because of being hung up on her ex or for whatever other reason is secondary. But if she actually was interested then "being hung up on the ex" wouldn't be an issue -- case in point, she doesn't turn down Ian when asked out, even though crazy ex is still around to be hung up on just the same.
He only attempts to blackmail Lena if she refuses.
"Only". A real upstanding citizen. Can't imagine why Lena could have any second thoughts about getting involved with such guy.
(i did have Lena refuse his request because that was her standard behavior regarding Robert to date, so, well, he didn't paint himself in a positive light there)
She questions her choice to have hooked up with him at first, but then admits that it scratched her itch and more they end up doing it, the more she enjoys it because he learns to satisfy her sexually.
Well, that's a wee bit different from "she only sees a relatively attractive college who scratches her itch", isn't it? If that was really all she saw, then what reasons would she have to question her choice?