Is the questioner sinful for what she did by taking the money her fiancé sent to her friend to buy a gift, instructing her to tell him that she bought the gift for the price she specified, and keeping the money for herself?

You and your friend have no right to take the money. She is an agent for purchasing the gift, and an agent is bound by the permission of their principal. Therefore, the money must be returned to its owner. It must be noted that the suitor is a stranger (non-mahram) to the fiancée until the legal marriage contract is concluded. It is not permissible for him to speak with her without a valid need, and the same applies to her friend if she is not a mahram to him.