Boraxo

I have a GLS 450 with 27k miles. Last week the brake warning light appeared. I took the vehicle to my trusted local mechanic who showed me that 3 of the brake pads were at 50% but the 4th (right rear) was down to 0% on the inside.
My mechanic said the cause was insufficient lubrication. As the car was serviced by my local Mercedes dealer <5k miles he said Mercedes might honor a claim.

Has anyone else encountered this problem or made a warranty claim based on shoddy dealer maintenance? I know brakes are considered wear items, but the dealer has an obligation to lubricate and check conditions during the B service.

Also any thoughts on whether I should replace the rotors? The left rear rotor has a nice groove up the middle and I am concerned that the new pad will squeak. But this would add $400 to my bill...