Q About Runnig Lean

What exactly would running really lean do? My friends LTR with basicaly same mods as mine (sig) thought he was running a custom map for the PC. When we hooked up to the lap top the map was blank (stock). We thought it might be running lean so that is why we checked. He ran it really hard for an entire weekend. It wasnt running great but we wrote it off as elevation change and bad gas. It still runs fine but did he do any long term damage to the motor?