Delta Mileage Upgrade Awards require at least a K or higher Main fare or a Premium Select fare. If they bought a L/U/T/X/V class economy fare, they will not be eligible for mileage upgrade awards. They also recently increased the mileage amounts required (I believe H/Q/K fares for US-EU are now 85K miles based on a recent quote someone posted). There is also no guarantee there will be upgrade inventory available. Most routes no longer offer Mileage Upgrade Awards, but flights ex-UK and ex-France are an exception as they can't process online seat upgrades from these origins due to additional premium cabin taxes. Other than ex-UK/ex-France flights, upgrades on Delta metal flights are now only available via online seat upgrade offers which have variable pricing (and may or may not be offered on any given flight).

Delta Mileage Upgrade Awards

https://www.delta.com/us/en/skymiles...rades-and-more