If you have a friend who loves gardening then why not give them a gift related to their favorite hobby? There are lots of gardening gifts so just find something that fits your budget and go with that.
If you have a tight budget, go for things like gloves, kneepads or even a fun hat. A pretty pot (or a watering-can) filled with a small bag of potting mix, a packet of bulbs, some gloves and a small trowel or other tool can be a great gift as well and don’t worry if they already have one, spares are always good to have on hand. Or consider making an array of hand tools (think of those kitchen utensil sets that go on countertops and use hand tools instead).
Or how about a subscription to a gardening magazine or website? If you have some time, you can create a list of websites that your friend can use for reference. Doing that research can take time but you will be giving them a great list of resources to draw from. You could take it a step further and create your own list of favorite articles, tips and tricks that you find and give them a print out (or save the paper and just put in together in a nice e-book format).
Your friend may have been longing for a specific plant that is hard to get. Maybe something you have to special order. Keep that in mind so you can time the gift appropriately.
If money is no object, then perhaps a more expensive tool that your friend has been without. A pull-trolley is easier to use than a wheelbarrow and, like some electric tools, is still not terribly expensive. Small electric tools such as whipper-snippers can retail for as little as $20.00. Or if your friend has a hose but not a hose reel, then that would be a more useful gift to keep them tangle free.
If you are going all out then automatic lawn mowers, electric cultivators, hedge trimmers and brush cutters might be an option for you. Just make sure to find out what they already have. If this is a gift they typically could never afford on their own, then it’s a great idea. Maybe you can split the price with another gift-giver. Imagine the look on the recipients face when they get a gift related to their favorite hobby. What a hero you’ll be.