I can understand you're feeling frustrating, because she is one of your costumers but she's doing something that it's not so nice. Sure, to buy something to reproduce it for relatives annd friends without the permission of the artist is not an honest thing to do, because at the same way people could sell those copies making money (and this is NOT honest). Well, your costumer said to you she reproduced some of your artworks to make a gift to a couple of friends or relatives and she's a good costumer? Well, you can say her (in a gentle way) that you're happy to know that, but you (like every artist) have to protect your works and so it's better if the next time she ask your permission. You can work on some digitalizations of your artwork to sell (that people can print just for personal use). Sure, everyone of us have similar issues, because we can't know if people use our works to print copies for others or (worse) to sell them, but we can "educate" people talking about it ;)