If the gift was given by the 6 year old and her mother, "formally," for example, at a birthday party, you should tell the other little girl that presents are presents, and no "give-backs" are allowed. But if it's a present that one six year old gave another six year old, informally, or just in the course of play, then "give backs" may be standard operating procedure. You should tell your daughter that maybe her friend doesn't understand the rules of gift giving, or maybe her friend only meant to lend her the toy. Buy her another one, if you can.