Cracked Granite Countertop

We had granite countertops installed in our kitchen in April this year. We have a corner undermount sink. Last month we had the walls painted and before they started taping, my 17 yr. old daughter said they scrutinized the whole countertop. They informed her of a hairline crack running from the sink to the outside edge, less than a centimeter from the angle cut to accomodate the corner sink. It is not readily noticable so we cannot honestly say it was not there before. How possible is it that the crack was started when the slab was cut for the sink and the angle, and is it common practice to reinforce a cut like that with a metal bar? The area in question measures 10 cm. wide, 3 cm. thick and a cabinet overhang of 7 cm. A representative from the granite company stated a metal bar is sometimes needed in cases like ours, and maybe they should have done that in the first place. The owner of the company has called to say that he cannot replace the section free of charge because there is the question of whether the painters caused it. He also stated it might be a challenge to match the new piece with the adjacent piece it will be bonded to. Do we have an argument regarding the possibility of the installer causing the crack and the painters just noticed it because they are trained to look closely for things like that? Thank you.