I would tell her that if he broke his tooth and the nerve root is showing, this is very painful for him and he may not be able to eat properly, thus starving to death and dying a long and miserable death. Guinea pigs must be able to get the proper dose of nutrients each day or they risk scurvy, which can cause bleeding in the muscles and tendons and excruciating pain all over their bodies. Additionally, they need to be able to eat roughage like hay nearly constantly to keep their very short intestinal tracts active: since they cannot benefit from a long intestinal tract in order to extract nutrients slowly while traveling through a long tube, they must eat large quantities frequently in order to make up the lack of nutrients absorbed.
That should help convince her. Just emphasize to her that by putting off a vet visit with a broken tooth that may require him being hand-fed, she may be condemning him to starve. See how that sits on her conscience.
Best of luck to your poor piggie!
