I also have software used by dentists for controlling the x-ray machine and displaying the image. The best I can say is dentists are about the cheapest clients around. I used to have several as clients and I finally gave up on them as they always wanted something for nothing or a discount on their bill, or refusing to update the equipment when needed or etc. etc. The software producers ARE goughing them, but its tit for tat.
I would just tell the dentist what the real solution is, and let him bite the bullet or do without.
One of my favorite dentist stories is the time I had a dentist bring his aircraft in for a avionics repair. This particular box had two units sharing the same case in the panel. One side was a NAVCOM, and the other was a DME (distance measuring equipment). Anyway the NAVCOM was not operating, so I repaired it and sent him on his way. Two or three months later he brought it back saying it was in_op again. I looked at it and the DME was not working, so I fixed that and when I handed him the bill he got angry and asked "didn't I garante my work, I told him sure, but the DME was out this time. He was still pissed, so I asked him if he worked on a cavity on the right side of my mouth, would he charge me if he had to fix another on the left side. He said yes, but that's different! My andwer to him was "it's the same mouth". He damed near turned purple he was so angry. Lawyers are almost as bad. I could tell some good stories there also.