I would not cross her off the list yet. I would talk to the person who did her teeth this year- that is a long time not to have teeth looked at and her bite could have shifted simply because of that. She was obviously in need of having teeth done, and half a bite off is not THAT far.
The dentist or vet, should be able to tell you why they were off, and I have watched many horses over the years that were off, get their teeth done and they came back on where they were supposed to be, but it does not happen over night.
If your friend really wants the mare, talk to the vet/dentist and get more info. The teeth were just done recently, so I would check into it. If they think it is something else, you can pass on her, but it may be worth a bit of your time to check on it.
I know someone who bought a young horse who was way off- much more than half a tooth. I believe she was a three year old. After some simple dental work for a couple of years of just keeping the bite even like it should be, the bite was close to being right where it should be. The farm she came from apparently does not bother to have their horses teeth checked (it's a large farm too that shows)