2010 GLK350 Harmonic Balancer
 

I have a 2010 GLK350 with 86K miles in which I have been the sole owner. I have performed all required maintenance. Last week, my harmonic balancer failed and, for want of a better term, ate the front timing cover and upper oil pan. My MB dealer tells me this should not have happened, but still wants me to pick up over 40 percent of the $6500 cost. Given I have performed all required maintenance, is this reasonable?

No, you shouldn't have to pay anything. That should be covered under powertrain warranty.
Not a part that normally fails.