What causes one brake to lock up?

[PHOENIX]

Ok this question is for a friend of mine who owns a F*rd (I think he would rather own a chevy truck, but he doesnt have money right now). But this question can be related to any disc brakes so I think its OK here.

Anyway he started hearing this rubbing sound coming from one of his front tires. He gets home and pulls the wheel off. He notices that he can spin one tire freely but not the other (both front). What do you think is causing this brake to lock up? My guess is something is wrong with the caliper.

I tried to call him right now to see if he got the caliper off and to see if the disc is still real stiff. When I talked to him last he was trying to get the caliper off, he was having hard time. Alot harder time then normal he said. If its not the brake caliper could it be another brake component? Im going to tell him to check the bearing. He just replaced the bearing not too long ago, so its probly not that. Now I wonder if he put enough grease on that bearing

Ohh well until I hear back from any comments are appreciated.

Thanks.