Writing a recommendation letter as an employer for a student applying to programs in a different field
In the letter, while you describe his duties, you could explain (and provide specific examples if possible) about skills that the former employee has developed and demonstrated that are transferable across all fields, including:


analytical skills
writing skills
interpersonal/collaborative skills
work ethic
initiative


Also consider if the person has written any published work, have they the potential for innovative research that could be a source of grant funding.