I think the correct answer would be to ask "why are they doing that and not using Google Sheets?".

There are a lot of good reasons for not using Google Sheets. Maybe the spreadsheet is using features that Google Sheet doesn't support, maybe one of the parties is in China, where Google services are blocked, maybe it is against company policy to use Google Docs, maybe they have limited connectivity.

It is good to acknowledge the obvious, off the shelf solutions, but if you are given a job, that's either because the customer did their homework and found out that no, it is indeed not appropriate, or, for some reason, they have money burning their pockets and they want a custom solution, just because. In both cases that's how you are getting paid. So, I don't consider "use Google Sheets, you idiot" to be an appropriate answer. Understand the customer specific needs, that's your job, even more so in the age of AI.

And maybe the interviewer will be honest and say "just assume you can't, this is just an exercise in software architecture".