The healing process of a bone includes multiplying certain cells in the bone. The area that is most affected by the multiplication is actually the area where the bone was broken. If the two parts of the broken bone are attached to each other, then the cells that multiply would produce intercellular substance that actually holds the cells together.
However, if the two ends are not close enough then the intercellular substance would not be able to fill in the gap. To keep the two ends firmly attached the doctors use metal rods or plates and keep the broken bone in plaster for several weeks when most of the healing process of the broken bone is expected to occur. This process might be helped by increasing the intake of calcium and phosphorus which are substances included in the bone structure.
However, this should be done with caution if the person has a previous history or family history of kidney stones. Applying certain pressure on the broken bone might keep the two pieces close but the amount of pressure should be determined only by a medical professional based on the X-ray images that show the healing progress.
As you can see, masturbation does not affect the healing process of broken bone.