remove it and replace, this means the stone too. bummer, try and do a bit more research on the person you entrust next time find out how and why they are qualified to touch youre home. the fact that this person started and then continued is baffling and now its too late.-best

@EHHP i did assume this was old and set for some time. gonna need mechanical means at this point IMHO.

unless it was just a day or so old.

still I would start with phosphouric acid and work up to low muriatic dilution with brass scrub brush and plan on etching the whole shebang.

Next time mix some earth into the portland grout to blend.....install & let stiffen, tool the joint and strike the overfill off "after it sets". No sponge or attempt to clean just yet..... clean all excess within 24 hrs.