Need a lot more detail than that. Any light left on will drain the battery, but GM also has a real lot of generator trouble and repeat failures are common unless you also replace the battery unless it is less than about two years old.

You didn't list the symptoms so it's impossible to know where and what to start testing. How was the generator tested, and what were the results? They should be tested for maximum output current, voltage, and "ripple". Those tests are usually inaccurate if the generator is removed from the engine and tested at a parts store.