Its their problem no matter the cause.
We have done hundreds of valve jobs and thousands of chains. It doesn't matter what happened since they were in control. Believe me a MB v8 with 120k needs neither a valve job or a chain. The only thing it realy needed was upper chain rails.
About 20 years ago I got my first taste of reality on this subject. We did a preventative chain and tensioner (no rails). About a month later the car had a ignition problem (maybe even caused by our work - the green wire from the distributor to controller was intermittantly shorting to the valve cover. During a time of deficient ignition the thing dieseled and ran in reverse direction for a moment. Happens quite frequently with such problems. The rails broke and while trying to start the car the valves on the left side bent.
So whose fault is it? Doesn't matter. We did the one and only free v8 valve job in the 25 years we ahve been in business.
There just isn't anyway to justify doing preventative maintenance that includes doing the work that was to be prevented. I am trying to point out that this is not a technical issue. The technical issue is moot. This is a business issue at this point. Fault has no part in it.
Bosch Master, ASE Master, L1
33 years MB technician