But then Professor Quirrell had also seen Harry taught Occlumency, he had taught Harry how to loseā¦ if the Defense Professor wanted to make some use of Harry Potter, it was a use that required a strengthened Harry Potter, not a weakened one. That was what it meant to be used by a friend, that they would want the use to make you stronger instead of weaker.
Which calls back to this bit in Chapter 51: