I thought I already responded to this, but I don't see it so maybe I imagined it. (Apparently my mental confusion is still with me!)
If I were your parents I would seriously consider getting them OUT!!!!
My wisdom teeth were taken out 20 years ago. I think two were coming in sideways, and I don't remember if the other was impacted. I was surprised to see the fourth one on the xray. I thought they were all pulled but there it still is!
Apparently the dentist can only tell if it needs cavitation surgery is if he goes in with a tiny drill bit and if its soft and mushy it needs to be cleaned out. Something like 90 or 95% of them need it. I'm really squeamish and couldn't really stand to read up on it, but I'm glad my dentist did mine. I'm also glad I didn't know in advance I was going to need it! Of everything he did the cavitations healed up the fastest and painlessly. And from what he said the cavitations are responsible for the arthritis, moreso than the amalgams. I don't remember them being that expensive. I hope you're wrong about the price!