With the engine idling steer the wheel all the way to the left or right and listen? What do you hear? The belt will almost certainly be slipping when you do that so don’t hold it more than a second or 2 intermittently.
What did they mess up ? And if they admitted it seems that they are willing to correct the mistake.
Mistakes happen .
They didn’t repair anything just refilled it with freon. Can that affect the belt?
Yes. If the A.C. compressor is cycling more now than when it was before the repair, a loose or worn belt will slip more now than before the repair.