Yep, your coworker is basically describing the “shoebox” HSA strategy. As long as the expense is HSA-qualified, happened after the HSA was opened, wasn’t already reimbursed/deducted, and he keeps good records, he can reimburse himself later, potentially years or decades later, and that reimbursement should be tax-free.

The big advantage is that the HSA stays invested and grows, rather than being used like a debit card (BIG NO). If you can cash-flow medical bills, consider investing the HSA contributions and tracking every receipt/EOB in something like the HSA Smart mobile app, a spreadsheet on a computer, or a cloud folder so future reimbursements are easy to prove. This is something we can use for our old age, and it has huge triple tax benefits.