Quite sure. Am looking at the reservation right now. You can't do it with BA Avios- which was why he had to book it using EI Avios, even though the fees are about $40/person higher. What was really odd was that the flight wasn't available as a saver award on United.com, but was available as a saver award on avios.com. I don't know how that is possible from an inventory perspective, but it was. I don't know how many routes they offer this possibility- this is on DUB-IAD, UA127, codeshared as EI6995.

Example: look at October 29, 2020 on avios.com, search DUB-IAD. You can see both the UA and EI operated flights on that day available as award tickets. And to add to my comment about the odd differences between united.com and avios.com, united.com shows 2 econ saver awards available on that United-operated flight. Avios.com shows 9+ econ saver awards.