After reading the FAQ i cant see why Ezren Cant play sleep to aid another character. The FAQ says "When you encounter a card, you—and only you—can go through the following steps. No one else can perform these steps for you, though other players might be able to play cards to help you deal with the encounter’s challenges. During each of these steps, you and the other players may perform only the specified actions. Players may only play cards or activate powers that relate to each step."
The sleep spell says "Discard this card to evade a monster whose highest difficulty is 10 or lower".
The word your or turn does not appear.