Inspect the repair invoice. There should be something there about the warranty limitations. Other things cans cause oil leaks. Let them inspect it and see what they say. Don't show up with an attitude, let them do the talking, ask questions then decide what to do next. If you don't believe them, take to an indy for a second opinion. Try to stay civil. You may be able to work something out even if it is outside of their official warranty limits. If it is the head gasket, it should not have failed so soon.
Good luck, Dan

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2012 E350 Bluetech