and NO, NOT stupid to use clear nail polish to keep the edges from fraying, that's actually brilliant!! especially for a little small cross stitch! I've seen people bind the edges of larger cross stitches with ribbon or bias tape, which is cute but can be a little wonky.
Your Nanna is going to love it...happy b-day and congratulations to her!
Tell her not to whoop it up too much with the liquor drinks