It does sound like the very common C5 problem of the handbrake mechanism sticking inside the caliper.
What was done to free off the sticking brake spreviously?
I have to say I have had little joy with finding a long term fix for the sticking handbrake mechanism. A full strip down and repair or a full replacement of the caliper, the latter is what I have been doing.
I had this problem on one of my C5s and very nearly set fire to the brakes, it was binding so badly. After that, until I had time to sort out the fitting of the new calipers and replacing the pretty burnt discs and pads, I drove it without using the handbrake. Not something I would recommend doing for long, obviously.
If you are thinking that the handbrake is binding the brakes, you should be able to pull it off using the lever on the caliper itself. With the wheel on full lock (or removed) reach infront of the wheel and locate the lever on the caliper (it has the handbrake lever attached to it ...). Pull the lever towards the front of the car, you might need pliers, and the brake will release. If the handbrake is not sticking then you will no be able to pull the lever, as it should sit in the most off position. Just don't try it with the handbrake applied

If it appears to be sticking and you have manually released it, see if you can avoid using the handbrake until the caliper is sorted.
I have to say, I question a garage who only fits new pads to one corner of the car.