2000 Chevrolet Malibu coolant system

Your friend is basing his guess on past experience. GM has had some problems with intake manifold gaskets, but that isn't really a serious repair. It should only take a couple of hours.

Before I would trust that kind of diagnosis, I'd want to verify that's really what's leaking. One method is to use a pressure tester to pressurize the cooling system, then you look for where the coolant is leaking out. If it really is leaking from the radiator, for a car as new as yours, that is commonly caused by not replacing the antifreeze often enough. GM uses the red "Dex-Cool" antifreeze. They advertised it as "lifetime" antifreeze to make their cost of maintenance appear to be lower than that of their competitors, then on the sticker on the car they said to replace it every three years. Even the Dex-Cool company says to replace it every two years because that's when the additives, like corrosion inhibitors and water pump lubricant, wear out. If you follow either of GM's printed recommendations, you'll have acids build up in the system that lead to corroded and leaking radiators and heater cores.

If the leak is too slow to see with a pressure test, you can add a small bottle of dark purple dye, then search later with a black light. The dye will show up as a yellow stain that you can follow back to the source. Auto parts stores will have the dye, and those that rent or borrow tools will have the black light.