The cancer is a low-grade lymphoma. The doctors don't know yet at what stage the lymphoma is in, but the good news is that lymphoma has one of the highest rates of recovery amongst cancers.
Apparently my mom has had the tumor for a couple of years. She has been complaining of pain in the left side of her jaw for quite some time, and she had wrongly been led to believe that the pain was caused by dental issues. I am concerned that she may have had this undetected for so long, but at the same time this would seem to indicate that this is a slow-growing tumor, which is a positive thing.
The doctors are going to run a battery of tests (blood work, CAT scan, etc.) to determine the extent of her cancer. If it's in stage one and hasn't spread to any other lymph nodes, treatment is just a matter of removing the tumor. I don't believe chemo is necessary in stage 2 either. Hopefully, the lymphoma is in one of the early stages, so as to minimize the pain and risk to her. We'll know more in a few days.
[Addendum: You should read my dad's recent post about pain.]