To unlock the Group LF Healer in Borderlands: Game of the Year Edition, you will need to revive a co-op partner when they are down.
You can do this when playing co-op. When your partner is down, run over to them and press
to revive them. This can take a few seconds, so make sure you kill any enemies in the area before attempting to revive them.
If you're playing solo, you will need to sign in a separate account on a second controller. Have some nearby enemies kill the second account and them revive them with your main account.
Once you have revived a co-op partner, the trophy will unlock.