lifesaver

Someone or something that provides crucial help in a difficult situation. The term is almost always applied to situations that are not literally life-threatening. You're a real lifesaver for coming over and fixing my flat tire. The knife I brought on the camping trip turned out to be a lifesaver when we realized we had forgotten a can opener.

you're a lifesaver

You are wonderfully helpful, especially in my time of need. You brought me coffee? Ah, you're a lifesaver. Thank you much for picking up the cupcakes on your way to the party—you're a lifesaver.