A sore that wont heal?

I am a bit concerned about a friend of mine. A few years back he got a sore on his lower bottom from sitting too much. Which did not heal and was treated as a pressure sore. It did heal. Recently he fell on his bottom (well not so recent a few months ago) and it now has a sore that won’t heal. It smells very strong. He has been to the Doctors and is due for surgery to remove the scar tissue and dead (?) tissue. He told me the surgery will fix it. But I have never heard of a sore not healing. I am concerned. I know this is not a medical site but any feedback appreciated since he cannot remember the name of the affliction for me to look it up. He tells me the Doctor assured him all is well.