If you cracked it, I don't think it can be fixed, other than maybe
laying another sheet of formica on top of the whole countertop, which
whould likely look like crap.

Ask the customer what would make them satisfied.

Is the crack really that bad? Jeezy Weezy - it's formica, not granite.

Maybe you can "fix" it by telling them you screwed up, you are sorry,
ask them what would make them happy, ad offer to knock some $$ of the
bill.

Most people are pretty forgiving if you are honest with em.

IMO.