1989 GMC Jimmy No oil in engine

Engine Mechanical problem
1989 GMC Jimmy Automatic 91000 miles

a local lube place did not put oil in my vehicle and I ran it for a short distance before realizing it and contacting the lube place they said if I had any problems they would take care of it and now after several months of driving the vehicle I have no oil pressure and they will not take care of it - they want me to prove to them it has damage and I can't do that unless I tear the engine down - I am taking the owner to small claims court - can you tell me what kind of damage it could do to a motor to run it without oil. Or any other information would be helpful. Thanks in advance!