Someone mentioned at a venue that you can choose to re-roll either one dice or both.

He missed his attack and rolled a seven (6 and 1), and made himself re-roll, but only grabbed the dice w/ the one and leaving the six on the map. He swears that you can do that, but we've never played that. In the rules it says you can force to reroll one or both dice, but I figuired that one meant for cases where you only roll one dice??

You are correct. Both dice have to be rerolled.

No, you must re-roll both dice.

Just show him the PAC next time he tries it:

PROBABILITY CONTROL (optional): Once during your turn, this character allows you to reroll one of your dice (or die) rolls and to ignore the result of the original roll. All dice used in the original roll must be rerolled. A character using this power must be within 10 squares of the character that made the original roll and have a clear line of fire to that character. A character may use this power on itself. In addition, once during your opponent’s turn, this character allows you to force that opponent to reroll one of their one dice (or die) rolls and to ignore the result of the original roll. A character using this power must be within 10 squares of the character that made the original roll and have a clear line of fire to that character. In multiplayer games, this power may be used only once when it is not your turn.

Thank you. I'm 99% sure I was right on that one too.