He first saves his receipt for the car wash and takes a picture now to show there is no sign warning of the danger. Then has a choice. He can write a demand letter and send it to the owner of the car wash -- who probably will say "not my fault"; or he can take it to a lawyer who will write a "LAWYER'S DEMAND LETTER" and suggest that unless the shop pay in full he will seek broader relief and represent multiple plaintiffs and want punitive damages -- unlikely but who knows; or he can go right to small claims court. But he would need to prove that excess pressure creates the damage in order to recover, and that would likely require an expert.
One other thought, contact the Better Business Bureau, and the local consumser hot line service at a local paper or radio or TV station.
This is intended as general information only, NOT legal advice. You are not my client and I have no obligation of any kind to you. To retain a lawyer I suggest you go to www.AttorneyPages.com.