Are there people below him on the standby list? How many? Can you see the seat map, for example on EF, which has the advantage of indicating blocked seats?

Was he able to complete OLCI and get a seat request card rather than a boarding pass?

I suspect that he's confirmed on the flight but standby for a seat assignment. If the flight really is oversold, as a kettle on an E fare, he's at risk for an IDB, but DL is likely to find volunteers unless there's some special event, etc.

It might be better to avoid checking a bag initially, but plan to do at the gate as he will be among the last to board. Also, he should avoid getting to the airport late but IME it's not necessary to go extremely early, although time should be allowed for his lack of familiarity with flying.