Being "Hit like Holyfield" means that the girl sleeps around a lot. She has been "hit" by many different guys.
The fighters that fought Holyfield typically took many punches and the fights lasted the full 12 rounds. There were "hit" many, many times. A girl that has been "hit like Holyfied" is probably not safe and also has STDs.
Brad: Damn, that girl over there is a dime.
Homewood: Hell yeah, but she has been hit like Holyfield so you know you will be at the free clinic on Monday wondering what is dripping from your cock if you settle for her tonight.