Friends always bought, sold, traded boots with each other. If you're getting a deal fuck it, send her.
On the one hand you might already have that boot stank, but I've worn rentals before and I didn't die. Actually took a few pairs of nordicas when we were upgrading our rental gear at one mountain. If theyre molded to somebodies feet they're molded to somebodies feet, but if they fit you alright and the price is good, I don't see the problem.
Probably throw some powder in them so your toes don't fall off though.