Hummers: 6.5 diesel truck, fly wheel, head gaskets

Question
the vehicle im asking about its not a hummer but it has a 6.5 diesel engine on it . well water is getting to the muffler the oil doesn't have water a mechanic told me it may be the head gaskets and his telling me we need to replace them he say that if water was to get inside to a piston  the combustion would be stronger and it could break the fly wheel and the starter this is a 1993 engine how truth is this

Answer
How much water is coming from the muffler? Is it condensation or actually coolant?

I recommend performing coolant pressure test. Use the instructions that come with the coolant pressure tester. Certain reading such as a bouncing needle indicate compression entering cooling system.

If the head gasket is leaking the mechanic is correct that additional damage could result if problem is not corrected; but not specifically just the flywheel or starter.