Your friend's new snake is acting aggressive more than likely because it is frightened and in a new situation. You guys need to leave it alone for about a week and let it adjust to the new smells and areas that it has been placed in. I wouldn't try to feed it for that week, just leave it alone and allow it to get used to everything.

After that week, try handling the snake a little bit. 10-15 minute intervals should be sufficient to get it used to you and being handled by you. Just keep in mind that the poor thing is scared and doesn't know what's going on. Give it time to adjust. Once it has settled in for the week, then try giving it some food. Don't hover the mouse in its face, try leaving it in the feeding tub (or if you feed in the tank/viv, leave it on a plate or paper towel) and leave the snake alone to eat.

Hope you get it figured out and all. Good luck with him!