I played a multiplayer game of magic today that had a very interesting turn. The game was a five player game with your allies being the player to your left and right and your opponents were the people across from you. On of my allies ended up getting a Wall of Resistance and would gain +1 toughness at the end of a turn after damage was dealt to it. I decided to attack my ally with this ability with a 1/1 creature to strengthen it, but the other ally said I couldn't due to it being an ally. Right after that, the same ally used Rootwater Hunter's tap ability to damage that same 1/1 creature and kill it.
To me, these two action seem equal and if one action can't be allowed then the other shouldn't. Since this was the pivoting point of the game I would like to the actual rules and how they apply to each situation. So can you attack an ally and can you target an ally with an effect that deals damage?