After a bad car accident my friend has developed a large hard lump in his leg just above his knee.
It means he can't bend his leg or walk properly.
He didn't break his leg but did fracture his pelvis.
He has been told it is bone growing into the muscle and it may reabsorb but is difficult to operate on as it will probably damage the muscle more.
Any suggestions? 

Anything to do with the joint or bone, consult a chiropractor.

It could be calcification.  I think this can happen to bones as a result of an injury.  See a chiropractor.  This happened to one of my dogs who had a bone lump on his wrist for a long time.  I took him to the veterinarian and they told me that is what it was. They told me that once calcification is present it won't go away.  I checked online for alternative remedies and found that taking omega 3 oils was supposed to reverse it.  I tried it in addition to some energy work (directed intention with my hands every night).  The bone lump is completely gone and the doctor was surprised. I think the oils were a big part of it.