If your mechanic is confused over what to do in this situation, really you need a more highly skilled mechanic. Any time the oil and coolant are mixing, the engine can be destroyed in a heart beat. This is an 'all hands on deck' situation for your vehicle. If there is even a question about the condition of the oil cooler, REPLACE IT and all involved lines, connectors etc. Also do a complete flush of you oil system and cooling system. This is a VERY delicate operation. Flushing the oil may have to be done with an external pump at first , so you don't do damage to the bearings in your engine. Replace the oil filter with EACH flush. Or, you will be replacing a $40,000 engine in no time.