If they're just casting the concrete without bonding with the previous concrete, I afraid it can't takes the load. I believe that your contractor can easily add additional steel bar to extend the counter top, what I think is right will be... drill hole to your existing counter top..put in additional steel bar..grout it..then only cast the extended concrete.. it should be having the bonding effect and able to carry the load.. Having a contractor like this is a big headache.. =.=

Good luck bro!