I agree with your parent. When I started with (Python) webdev some years ago I picked Flask, but nowadays I use Django, as it has superb documentation and useful things built-in. In particular I miss the user-management, authentication and authorization in Flask.

Personally I consider Django without any additional apps less complex than Flask with apps (or "plugins" in Flask jargon) to handle basic stuff like auth.

If performance is a concern, both of them are insufficient and I would rather pick another language like Rust, Go or even Java.