do somebody good
1. To have a positive effect on someone. In this usage, a noun or pronoun is used between "do" and "good." If you're feeling sad, getting out in the sunlight should do you good.
2. To focus one's energy on charitable acts. In this usage, "do good" is a set phrase. Monica has always been one to do good, so I'm not surprised to hear that she joined the Peace Corps.
See also: good