It’s really hard to buy for stoners because the only thing they like is illegal to buy! If you think outside the box, there are actually plenty of things to buy your friend that are safe and legal! Just think about other aspects of their life and how they could be affected by their marijuana habit. For example, almost any stoner would appreciate a shit ton of food! Here are some non perishable options!

A bracelet that is secretly a pipe! I would start smoking weed just to use this. You can usually find this for under $20 here.
Socks are always a great gift for stoners because we are forgetful and lose them all the time(just like regular people) ! They also double as storage sometimes if we need a less obvious escape. You can get five pairs here for around $10.
There are tons of secret stash containers on the scene, but this one takes the cake for me because they are always food or beverage cases. Whatever you do, make sure that it is weighted like the real object it is mimicking; a real real light closed Red Bull gonna give you wings to jail. Here, you can find this for under $15, but there are other secret stash containers as low as $5
nectar collector
If your friend is interested in concentrates, this is a must! You can find it here for under $20, which is much less than a Nectar Collector! Though this does not have a percolator or metal durable tip, it does the job and is so handy to have around for concentrates. Better than pulling out the old rig because you just heat the tip and suck the concentrate up with it! It vaporizes on contact, which provides a smooth smoke. Use it on a heat safe surface of course.
Stealth, compact, and easy to use Exhale smoke into the chamber and odorless air comes out the other end Smoke on-the-go
This does come in different colors just so you know and even has a little key chain! This is basically a professional sploof that you exhale in to let out clean smoke. Anyone can benefit from that, and it’s under $13 here.
2 piece grinders are extremely self-explanatory to use: place some bud in (try to avoid placing dead center), twist 5-10 times and gently shake for a couple seconds while still closed. Open it with the bottom rounded part in the palm of your hand and carefully turn over where you want to pour your materials onto (i.e. a clean sheet of paper, your mason jar lid upside down, your washing machine, etc), then lightly tap to help get the ground material out.  If some material is still stuck, simply re-close and twist in both directions again, open and tap the pieces together to dislodge the material.
Though owning a grinder already takes out a lot of the work for us, imagine how much easier it would be with a crank on it! Good grinders that have ash catchers are a necessity, but the crank is just luxury. Find it for under $15 here.
These are awesome because you can actually move the flame where it needs to be instead of flicking your Bic around forever. They’re under $6 here, though they can be as low as cents each in bulk.
Not into the flames of the last lighter? This one is USB rechargable and flameless, which is a plus for clumsy stoners as well as economical ones. Here, they are under $13 though can be found much lower.
If your friend constantly loses or steals your lighters, a hilarious gift as well as useful would be to buy them 100 lighters! They are under $20 here, making them less than a quarter each. If you have a lot of people you need to gift, maybe just give them each one of these with a nice decoration on it.
There are sprays to get rid of cannabis smell as well, but how classy is this candle here? It would fit in with any stoner decor and kills marijuana odor. So replace all your candles with these! You can find them here for under $15.