Well, this HAS happened to me...and you do have to keep the attitude, "once its theirs....its theirs to do with as they please"
My sister put the quilt I made for her (hand embroidered with their names and date they married, along with names and dates of birth of their two children with embroidered babies, wedding rings etc....) and I went to their house only to find it on their bed, where it belonged, BUT with THREE huge dogs laying on the bed on top of the quilt and it was FILTHY with hair, and paw prints.....I was SICK!!!
But, didn't say a word....just let it go and yes, I have given her other things but never anything I put so much work into as I did that quilt....
SO....I feel for you....its wrong but there really isn't anything you can do or say to them that will make you feel any better.