With the current bikes with indexed shifting, the drivetrains Do indeed wear together. Normally you would replace a chain and a cassette or freewheel. However, if the bike was old enough to not have indexed shifting(each gear clicks into place) normally the chains and freewheels wear for quite awhile, and could be replaced replaced separately... although not always the case.

I do indeed hope that the shop has the orginal bike... keep us posted!