I cannot finish this quest because i already opened the safe in the dino lobby thing and took everything out of it and it wont let me find the evidence thats in there as i already have it so i cannot finish it..

You can only finish that quest during nighttime. Wait til it's dark and go talk to the nice lady.

Okay... so you have the evidence... Have you talked to the guilty party around midnight (in game)? Since Boone is in his sniping position at night you can only lead the person to him during his shift at night. Also, the brahmin killer is at the corral at midnght.