To me if Eugenie is not a jewelry person or has no great affection for her wedding tiara, then it makes no sense to gift her her wedding tiara. But if Eugenie is a jewelry person and/or if wants to gaze lovingly at it, show it to her children and grandchildren while, pardon the pun, regaling them with stories of her wedding day, then I don't have a problem with that particular tiara being gifted/bequethed to her.
While absolutely lovely, the Greville Emerald Kokoshnik Tiara has no royal provenance other than being worn by Eugenie on her wedding day. Yes there is a very real likelihood that one of Eugenie's descendants will sell the tiara but that is not the same loss as say with Queen Victoria's Sapphire Coronet. The Sapphire Coronet was designed by Prince Albert for Queen Victoria, Victoria is depicted wearing that tiara in a Winterhalter painting and wore the tiara to open parliament, the first time she opened Parliament since Albert's death a few years earlier.