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’d probably let them know I need a little bit of prep time, and then print off a one page RPG like Honey Heist. They’re quick to start, and a great way for people to try TTRPGs for the first time. After that, if they decide they like roleplaying, we can get into something more serious