Echoing others, although having 500k of debt is horrifying, it's also not completely out of the norm for a new doc that went through the specialty position training. The idea that this would be held against someone is, frankly, pretty cruel (and strategically, as others said, probably not rational either, because you will own this person if you hire them, at least until the loans are paid off).
I will say, though, that it's not something he should have just casually mentioned.