Buying for someone whose practice you do not share is genuinely difficult, because the things that look most obviously right in a shop are often the things they already have strong opinions about. The reliable rule is to buy the consumables and leave the tools alone.
Buy what gets used up
Candles, incense, resins, herbs, oils, charcoal. These are the things a practice consumes constantly and nobody minds having more of. They carry no assumptions about how someone works, and if the scent is not to their taste it is a small loss rather than an object they now have to keep out of politeness.
Quality is where to spend. Better incense, a resin they would not buy for themselves, a properly made beeswax candle — the upgrade is noticeable in a way that a novelty item is not.
What to be careful with
Tarot and oracle decks are personal to the point of being awkward. Art style matters enormously to readers and there is no way to guess it. A gift voucher or a box for the decks they already own is safer than a deck.
Ritual tools — athames, chalices, wands — are chosen deliberately in most traditions, sometimes with rules about who may give them. A beautiful one you picked may sit in a drawer for reasons that have nothing to do with your taste.
Crystals are safer but not free of this. If you know they work with a specific stone, good. If you are guessing, a nicely cut piece of clear quartz is welcome almost universally.
The joke problem
Anything with a slogan about being a witch is a gift about how you see them, not about what they do. Some people love it. Many find it slightly diminishing of something they take seriously. If you are not sure which they are, do not risk it.
Practical things that get quietly appreciated
A heatproof dish for burning. Charcoal discs, which people always run out of. A decent lighter. Small jars. A cloth for the altar or the cards. None of it is exciting to hand over and all of it gets used weekly.
We have written longer versions of this in a gift guide for the witch in your life and crystal gifts under £20 that do not look it.