Car brakes worn down only on one side?

Your alignment might be off.

Could also be a break piston in need of adjusting.

Alignment would affect tire wear, sounds like one of the calipers or both need to be rebuilt or replaced.

Driver’s side may not be retracting after applying brakes or passenger side is not working !

Re – Bleed your brake system, add new fluid…... could be air in your lines :-/

Maybe your car walks on the sides of it’s feet?

Or errr… You need an alignment?

Maybe the mechanic forgot this wheels brakes, when he installed the other three? Its happened before.

In my experience this is usually the caliper sticking, but it could be pretty much anything listed above.

Have a reputable mechanic check it out. Does the car pull to that side while braking? The caliper piston could be seized on the passenger side making that brake inoperable.
Normally, the brakes on the side of the car closest to the side of the road (not the center of the road) wear faster.