The federal Credit Practices Rule only provides for administrative enforcement but you need to look to your state law to see if an unfair or deceptive practice is grounds for avoidance of the contract. If your state's law would provide that type remedy and you then collected from the cosigner (knowing you should not collect) the cosigner might assert later, in an action to recover what you collected, various intentional torts that could open the door to punitive damages. Much would depend on how egregious your bank's conduct was. I would be very cautious before trying to collect from a cosigner who was not properly notified.
My opinions are not legal advice and are worth what you paid for them.