'reset' Seatbelt?

I've never heard of that but if there was a release it would be on the rewind mechanism under the trim panel. You need to unbolt the seatbelt anchor, take out the lower rear seat section, you might need to remove the upper package tray but I can't remember. you need to remove the top trim above the rear seats by folding down the rears and finding the 3 bolts, take the upper trim piece off where the seatbelt goes through, remove the rocker panel cover and then pull the lower trim piece off. Under there you will find the seat belt winder mechanism.

It sounds like the belt got twisted and then rolled up and is now stuck.

Tell your girlfriend to stop being a goofball and put her seatbelt on like a normal adult. I understand wanting a little extra slack but she doesn't need to pull the damn thing out all the way (unless she is that big.....)