The fact that she’s frugal says only that she has no intention of spending her own money and expects others to spend their money on her so she doesn’t have to. I don’t know why you keep seeing that as a point in her favor or somehow a statement about her ability to take from others. She’s only frugal with her own money. She seems quite liberal about taking anyone else’s. And I wouldn’t doubt for a minute that she still has no immediate intention of breaking up with the boyfriend. Depends on how shiny his next dangling carrot is.
Be nice til you get your calculator and shirt back. Promise her a nice dinner, ask her to bring the stuff, then take her to the finest McDonald’s you can find. Problem solved.
ETA; Sorry you had the experience and glad you realize you need to let go.