If all they did was replace the pads then I suspect he may have damaged the piston seal when compressing the caliper piston when putting the new brakes on.
When you replace the pads, you have to retract the piston in order to fit the new pads in the caliper. If the piston was far enough out, when he pressed it back in, it could have damaged this square cut seal which allows the piston to relax when you are not pressing the brakes. When this is damaged, the piston may not relax when you let go of the brakes which would keep the brakes applied on that caliper.
All this is clearly speculation as it is possible that this was a coincidence.
If this were me, I would charge you what I paid for the part and do the labor for free. Not because I would accept blame (unless I knew I did something) but just because of the way it looks.
Let me know if you have questions. Thanks
Wednesday, September 30th, 2020 AT 4:05 PM