It seems like, RAW, if a Divination Wizard is in the party then the DM would have to ask that player whether they want to interrupt the roll … on every single roll that any NPC makes.
That's going to be really tedious 🙁 It could also sometimes include rolls that the players might not otherwise know had been made.
Clearly allowing the Wizard to make the call after the roll is known would be hugely broken, as would making a judgement as a DM as to whether they likely want to use it.
How do DMs normally handle that?
Is there any balanced way to apply Portent after the initial roll?