He wanted a reference for this and I couldn't find one either way.
The reference is in the limitations section of your manuals.
Wording would probably be "
landing (wheels touching the runway) max tailwind component xx kts"
Not "
flying" at 500 feet or 200 or 50.
Since we are not fortune tellers, the only way to know what the wind would be at landing (an event not yet taken place), is to rely on the reported wind from tower
Only after the landing has taken place are we able to check if it was actually within limits (how were we supposed to know before hand). To me it's like continuing beyond the outer marker if the visibility drops below minimum, we can continue, but it's not certain we can land. Same with the wind, if tower reports tailwind exceeding the limit, you have to go-around - otherwise you can continue and have a "look" if landing is possible (use of common sense/airmanship).
...and of course you checked your landing performance before hand if limited to check your margin.
I guess you already see it the same way, the above was written as argumentation to use with your F/O. If he is stubborn, ask for a reference where it's written that the ND wind should be used while still flying above the runway.