First thing I would suggest is to tell your cousin to not drive the car. If it is overheating it is possible to warp the heads or do more damage. If you can't figure it out maybe you need to take it to a repair shop for a diagnostic.

With that said, I would do a pressure check on the cooling system and the radiator cap. You can rent the testing tools from autozone. If the system is leaking or cap isn't holding pressure then that can cause it to overheat. One of the most common areas for that engine to leak are the lower intake gaskets. They weep coolant when bad allowing air in. These are expensive to fix as the rockers have to come out along with the pushrods as they go thru the lower intake manifold. Stupid design. At work when I see these leaks most customers chose to sell the car vs pay the labor to have it fixed.

If the car passes the radiator check then I would do a reverse flush of the cooling system to push out any blockage and then refill with proper coolant water mix. I know my 96 z3 also had bleeder screws in the fire wall for the heater core. Yours might too. If not Presstone sells a flush fill kit that you can install on the heater core line. This gives you a very high location to bleed all the air out.

I hope this helps.

 

how can I tell if I have lower intake manifold blown on a 2003 m.c. 3.4

 

Get a radiator pressure tester (loan a tool from the auto parts store), engine off, pump the pressure to about 13lbs. Listen and look for coolant to bubble out some where. On the 3100/3400, it bubbles on the left or right side (where the intake meets the head).