If you get that humidity up enough, the egg may fill back out. If not, it'll probably still be ok. The main thing is to prevent further collapse. I had a clutch that I couldn't seem to get the humidity high enough, and all the eggs in the clutch collapsed more than they should have. I cut the eggs a couple days ago, and the snakes were still alive and well. They should be coming out any time.
Have you candled the eggs? Are you sure that the one that is dimpling is fertile?
Steve