At the moment, you can’t transfer photos to a stick directly, except if it’s one of those which can connect to the lightning port at one end, and USB at the other end. There are quite a few of these. I use an iBridge with the app that belongs to it, and it works well.
If your friend doesn’t want to wait for iOS 13, which will change a few things, there’s also the option to get an Airstash, and transfer the images to it (wirelessly): Airstash Wireless USB