PE firms are notorious for paying peanuts and often a lot of comp. is not cash but stock based. Regardless, the cofounder cannot sell without your approval and if you indeed had equity, you can sue both the cofounder and the PE firm as they didnt do their due diligence correctly. PE firms usually wont want to assume that risk so I am surprised that they missed this.
TL;DR: Get a Lawyer. You cannot do anything else.