passenger rear wheel locked

if it were the e-brake cable he would not have gotten the drum off. Certainly, not easily, as stated.

With the drum still off, and in neutral. Be safe make sure car is on stands, choked, and stable. If it has a sure grip rear you need to make sure the other rear wheel is off the ground or it will not turn. If open rear it is of little matter.

You need to take a breaker bar or something and try to turn the axle, with the drum off, and see if it turns freely.
If not, it could be a bad axle bearing. I would pull the axle at that point as it is fairly easy to remove and inspect. If it is deeper in the differential, carrier bearing, spider gear etc. The axles will need to be removed anyhow.

But at this point more information from the poster would help. Like where you driving when it happened, type of rear 7.25, 8.75, open or sure grip differential?