if he does say something, just point-blank deny it. And make sure you are very upset and offended by his assumption which will direct the attention back on him for being such an @55**e. He can't *prove* you've had an hair transplant.
If he tries to get a closer look, you need to kinda go ... 'wow wow, what the *f**** are you doing man?' Stand up for yourself, dude. Ok, he's your girls dad, but put that aside and just treat him like a normal guy.
Your girlfriend should back you up if this situation occurs. You can tell her later (if you wish), claiming that it's a deeply personal subject and not the kinda thing you'd admit in front of anyone/everyone.