Greg is a friend of Rick.
Greg, Rick, and Vince were on the ferry that was in a crash. They helped many other people get out before Rick was trapped between two cars. Vince left to get help and brought back Izzie Stevens. Vince and Greg helped her treat Rick as best she could. Greg was then sent to get Search and Rescue to get Rick out, but he came back alone, saying that Search and Rescue had many emergent patients that needed help. ("Walk on Water")
Greg and Vince stayed by Rick's side while Izzie drilled several burr holes to relieve the pressure. Rick was then able to be rescued and taken to the hospital. ("Drowning on Dry Land")