Writing a recommendation for highschool student I am a full time researcher at a research institute, we are part of a university, but the institute does not have students, just researchers. I mostly have assistant researchers and interns that are undergrad. I am thinking of bringing on a final year highschool student who is really promising and has been winning some awards.

They are interested in me writing a recommendation letter (to US schools, but we are outside US). Is this advisable?

My main concerns are the appropriateness of a research based letter for undergrad admissions, as well as not being a Professor or Senior faculty as mentioned in questions specific for graduate school.  However when I think about myself, I am pretty sure I had high school teachers write my recommendations, non of which had a PHD nor, obviously, were professors.
You're giving the student an extraordinary experience compared to her fellow applicants, so it's natural for her to ask you for a letter explaining that experience and your opinions of her work.  And I think it's very reasonable to provide one.