W-2 employees do not get to deduct any work-related expenses. (Since the new tax law changes that started with 2018)
An employer may pay him per-diem for meals but there is nothing on the tax return for him. You may find that you will have to explain this if they did not learn the new rules last year.
Pub 463 https://www.irs.gov/pub/irs-pdf/p463.pdf explains what things are deductible....but TCJA took away unreimbursed employee business expenses for federal purposes, so unless your client lvies in a state that still allows them, he's out of luck.