I have nice beautiful oval fingernails.
Well, OK, most folks' ovals are longer than they are wide. Mine are about twice as wide as they are long. And most people have a kind of strip of white between where the nail bed ends and where the fingernail ends. And most folks have nails that go out at least a millimeter beyond the end of their finger. Mine stop about 3-4 millimeters before the end of the finger. I have fingerprint-whorls on the skin on the top of my fingers, the skin between end of nail and end of finger. My nail beds (what's left of them) go all the way to where the fingernails end, and a good portion of the time a bit beyond (drying out and dying & retreating).
My teeth are shortened and flattened on the ends. If you look at the ends they look like tree stumps with rings.
It's probably the toenails that do the worst damage, they're so much thicker than fingernails. Also, the biting angle is more awkward.
Oh, and to answer your question: nothing you can do unless your friend wishes to quit biting. Lots of us have no interest in stopping and you won't make a dent in the behavior if that's the case here.
Well, OK, I suppose you could offer to finance the surgical removal of the nail-remnants. That would probably end it.