my secret santa is an avid backpacker, need a $25 gift, not sure what would be best

This is a little over $25 but I really like my Petzl e+lite headlamp. It's 29.99 at REI.

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

Smartwool socks - you can't own too many pair. Or a merino wool balaclava or neck gaiter.

So many gear choices depend on the particular style and approach --- i.e., is this person a zealous ultralight extremist or a "take it slow and enjoy my campsite for hours" person who is happy to pack a lot of creature comforts?

Maybe a hiking/backpacking related book would be a good choice. Or a gift certificate.

*small first aid kit
*pocket knife
*trail map (if you know his/her favorite locations)
*camp towel
*trowel
*star guide for night time viewing
*tweezers with attached magnifying glass
*spray to make maps water resistant

A gift certificate is the safe way to go. If he (she) is avid, that signals very specific tastes and needs.

Or just be upfront - "Look, I want to get you something nice - what works for you?"

I did that with the first Xmas gift for my wife - getting her the specific compass she wanted, an aluminum, map grade Silva - the extra was the message I had engraved on the side...