The person may have everything needed from local and common sources, but there may be books or tools a gardener would like to own. If the gardener is just beginning, then plants may be a good choice, but books and tools are still good ideas. Talk to the gardener and ask to see the person's library and tool collection. Then, slyly, ask what tools the gardener is planning to buy in the future. Or, talk to your friend's spouse about any ideas for your purchase.
Books are an excellent choice.
If the gardener is really interested in the correct names for plants and the science of plants, there are good books in a range of prices to consider. There are also books with beautiful photos that the gardener can peruse as they dream of their future garden. These books can also be used to encourage and teach new gardeners. Most gardeners like to share their hobby and grow new gardeners. There are numerous tools for gardeners - the basic ones that most gardeners already have and then specialized tools that they plan to purchase.