If she spends that much time in the kitchen, the best thing is to think about items she'll actually use every day and that make her work easier. A good set of chef's knives (doesn't have to be the most expensive brand, but they should be quality) is something anyone who really cooks values so much. There are also precise digital scales, if she doesn't have a good one; kitchen thermometers that go into meat or desserts so you know the exact doneness; or even a good baking sheet or specialized molds for whatever she makes most. If she already has plenty of kitchenware, then look toward things she'll have on her while cooking.
A decent apron (the kind that doesn't stain on first use), a set of pretty and functional kitchen towels, or protective sleeves for when she's preparing things that splatter a lot can be practical touches she'll use every time she gets to work. Even a good pair of comfortable sneakers or clogs for the kitchen, where she spends hours, is something she'll really appreciate. If you want something a bit more special, there are recipe books by well-known authors, online courses on specific techniques (pastry, artisanal breads, etc.), or even a subscription to a cooking platform if she likes learning new things.
What I'd do is ask her casually if she's missing anything in the kitchen or if there's any technique she'd like to master better. That way you know exactly what she'll use and it'll be a sure hit.