Do you have anything prepared for this? Do you say, no but come back in a week?
Do you say, yes, here are four pre-gen character sheets, let’s play a random short one-shot?
Do you say, no we’d need to spend time prepping for D&D but here’s a simpler tabletop game which uses minis and dice, let’s play this right now instead? (for example, Shadowdark)
Or something else?
I would pull out a pre-gen adventure I’m familiar with, spend a little time talking about character creation (for all its flaws I actually think how the starter kit does it with a little set of pre-gen characters they can pick from is a really good way), and just sort of wing it keeping it a little bit light on rules and big emphasis on having a strong start + giving them freedom to fuck around / not expect them to stick purely to the DND mold of behavior. I’ve had really good results with this though, as long as it’s an exciting world for them I could see it going really well.