If you meant the gameplay fight against the bandits outside the ruins, you cannot cast more than one spell that uses up a standard action in the same round (in normal conditions, you can cast a bonus action spell, like healing word or shield of faith, and a cantrip, for example) so I think the mage just used up his action casting grease so he cannot cast another spell until later. As far as I know cantrips have unlimited uses, and the MC casted mage´s hand several times.
PD: The confirmation that ritual magic are not available in bG3 was a big hit, indeed. =(
It was that fight, but Swen seemed to use his fire bolt on a water surface while out of combat, around the time he also showed off feather fall, and then his talk track suggested he didn't have a fire bolt to set light to to his grease spell. Maybe I misunderstood and he just meant he had to wait a round, but did not get the chance as he was downed quite quickly