Definitely up to the dungeon master, however if say a player were distracting a guard and another player walks up behind said guard and stabs him in the neck. I would allow the guard a perception roll to see if they know the other player is behind them. If they fail, surprise happens as normal because I would not say the distracting player is seen as a threat. If they succeed then obviously they turn around and say what are you doing and may get a few insight rolls based on circumstance.