Ruger's single action revolvers have the front of the cylinder hand engraved with the last three digits of a particular gun's serial number. For instance, both my Bearcat and Single Six have the front of their cylinders engraved with three numbers that match the last three digits on their frames, respectively.
This is because the cylinder is fit to the frame and barrel of each gun. Which means the length of the cylinder could be a few thousandths off at the center pin mount or the length of the cylinder where it meets the barrel's forcing cone with your friend's Bearcat.
If your friend's Bearcat has a three digit number that does not match the frame number, then a mismatch has occurred sometime after the gun left the Ruger factory.
So, assuming that the cylinder and frame are from the same era Bearcat, perhaps the current cylinder is just a few thousandths too long for that gun. Which should be able to be fixed by a good gunsmith by adjusting the length of the forcing cone or the cylinder's center pin protrusion (called a bushing, I think).