I thought discs and pads were supplied in pairs. If this is true then either the garage did no work on your car or they are stuck with one disc and one set of pads that they will never sell on.
If you have to pump the brakes and considering that the caliper was replaced then it is likely that they have not been bled properly. However, just replacing one disc and one set of pads is most likely to result with unbalanced braking causing a pull to one side or the other.
As others have said, do not drive this car with anything other than the most care and only gentle braking. You could be putting yourself and other lives at risk.