If your backpacker heads out when it's cold then those orange "Hot Hands" packets are nice. Gloves are good too. Wool socks are good.
The LED Headlamps are always nice, and so are small LED flashlights. Batteries to go with them are good.
For $25 you might be able to put together a little gift bag with several of those items.
All these things wear out or get used up, so I think they make good gifts because I'm always puckering up when I have to buy them for myself