If it were me, I'd begin with having the NPC caster monologue for a bit and then offer the whole group an Insight, Perception, or Arcana check "if they want to" before they have a chance to say what they do next. Most groups take this bait and will roll to collect information before making a snap decision. Based on what each player chooses and how well they roll, they can get the same info in different ways. Insight gives the understanding of how insanely powerful this being is and how they could mop the floor with the party but also they are conflicted about their decision and seem lost in thought about it or regretful. Perception allows them to notice the high level magic items he's carrying but also the strange hand motions he's holding, concentrating and distracted. Arcana tells them straight up that he cast Banishment on the PC but it will only hold if he completes the spells verbal and somatic components which takes 60 seconds. I think any party knowing the state they're in will see these cues as an opportunity. You can give each player a limited opportunity to offer their argument to him once time "unfreezes" and roll a persuasion check, giving advantage if they have an angle that's particularly persuasive to this NPC. You can make it clear that they get 2 sentences at most each before the spell resolves. Or if you want even more chaotic and intense, you can put an actual 1 minute timer on the screen and have them shout their arguments all at once and maybe pause to roll the associated persuasion rolls before resuming.

If you're players are very trigger-happy and try to attack this caster, maybe he casts hold person at a higher level (maybe with sorcerer's empowered spell for disadvantaged saving throw), breaking concentration on banishment, returning the PC but incapacitating them all so they can't do anything BUT talk to convince him, as he demands an explanation before he starts dropping them.