Hrmmm. Sounds above novice. Does he have a clear ice system? (not sure if that is practical for you). Maybe some bitters to experiment with (I'm always surprised at how much they cost!) . Maybe some amari but I couldn't recommend based on his taste. Amari are quite different from one another and some tastes are....polarizing.

Other than the usual barspoon, mixing glasses, Boston shaker, etc , but he probably has all of that.

I jrecently bought:

1. a bar mat - https://smile.amazon.com/dp/B0773GQ4KD?psc=1&ref=ppx_yo2ov_dt_b_product_details
2. drink umbrellas for tiki drinks - https://smile.amazon.com/dp/B0936BBGMY?psc=1&ref=ppx_yo2ov_dt_b_product_details
3. skull reusable cocktail picks - https://smile.amazon.com/dp/B0BDGBBX69?psc=1&ref=ppx_yo2ov_dt_b_product_details
4. channel knife (haven't used yet though) - https://smile.amazon.com/dp/B00V6GFYNK?psc=1&ref=ppx_yo2ov_dt_b_product_details
5. Lewis bag and mallet for crushing ice - https://smile.amazon.com/dp/B089GKG2QK?psc=1&ref=ppx_yo2ov_dt_b_product_details
6. dropper bottles for tinctures, solutions, etc - https://smile.amazon.com/dp/B07W8Y5HYR?psc=1&ref=ppx_yo2ov_dt_b_product_details
7. and some butterfly pea flowers to steep in gin,

Hopefully that gives you some ideas.