They been known to deliberately break a collar bone, for example, to get someone out of a squeeze.
I have seen this assertion before, and I must say I am profoundly skeptical. There are good accounts of several notable cave rescues that have involved tight squeezes, that were quite challenging for the victim and the rescuers, but I have never heard of anyone deliberately breaking bones. Such a procedure could easily create complications that would worsen the situation.
I also doubt that it would do any good. For males, the critical dimension is chest thickness, front to back. Some cavers squeeze through by exhaling, pushing forward, and then breathing. That is about where I draw the line on forward progress. For women, the pelvic area and hips are the critical areas. One can generally slip the shoulders through by extending one arm and trailing the other. You need to plan ahead to decide which hand you want forward.
Of course, this does get more complicated with a victim who is already injured. In one situation, we were able to bring the victim to a squeeze, whereupon we took him off the stretcher, gently wiggled him through the squeeze, and then placed him on another stretcher. That was dicey enough, because we were all concerned about occult fractures and injuries. I believe it took us a good hour to progress about three feet.