You have to use epoxy resin tinted to match the limestone color. Best way to repair that, you will have to get a syringe and fill the crack solid with epoxy starting and back-end and move forward. If the crack is to narrow and the syringe needle cannot fit, you can use a grinder with a crack chaser blade and create a fake joint... Being this crack more or less in a straight line, it will look like a joint.

I would get someone who can make this repairs, this way it will be done the right way, talk to the contractor who cracked the stone maybe he can do this sort of repair himself or maybe he knows someone this way you get a better deal if he is not responsible for that.

Good luck