Yes, if he was not reimbursed for meals, you can claim the Standard Meal Allowance if he was away from his Tax Home overnight.

But base his income on his records, not 1099s.

Are you sure he has a tax home?  Itinerants can't claim either lodging or meals.  

TP has a tax home here in WA.

What is the business connection with WA?  What are the personal connections? What are the duplicate expenses? 

They maintain a residence here in WA. Actually, live with their parents. Return back here, between jobs.  

If he is reimbursed on a 1099-NEC for meals on a per diem basis (whatever rate that is for company), then he should be allowed to deduct the Standard Per Diem on his Schedule C.

He shouldn't have to pay income tax on a reimbursement.

Correct?

Sounds a lot like the case that Austin Yanke lost in Tax Court.  I would post the link but it's a mile long, and trying to shorten it didn't work.  Do some research on "tax home" before signing the return.  Since you're in the 9th Circuit, look at those cases in particular (although none of them seem very consistent).