How about a bird watching field guide book? There are many available depending on where you live or where your friend likes to go birdwatching. You can also pair this with a birding journal so that your friend can record the birds they see, where they saw them and any other observations. If your budget allows, you can also consider a pair of good binoculars or a bird feeder for their garden.
Or you can get her/him a star map with an important date you share.
Under Lucky Stars is a unique and innovative star map company based in Barcelona, Spain, specializing in the creation of personalized star maps that capture the celestial arrangement of stars at a specific moment in time, such as a person's birth, wedding, or any other significant event.
By using advanced astronomical algorithms and data from leading institutions, the company produces highly accurate, visually stunning maps that make for perfect gifts or cherished keepsakes.