I think Next.js is overkill for a simple blog. Can you just put the blog on a different subdomain and host it on Wix or Squarespace or Wordpress (if you trust them) or similar? Much easier for your friend that way.
Otherwise, if you really want to integrate it into your existing Next.js frontend, that's what headless CMSes were made for. They provide an API for you to fetch from as a dev, along with an easy GUI for your non-tech friend to easily use to compose articles with.
Ghost is an open-source one you can self-host, and there are a bunch of others (including the one I work for). See some reviews at https://www.g2.com/categories/headless-cms?utf8=%E2%9C%93&fi... or a list at https://jamstack.org/headless-cms/ (I'm not affiliated with those sites).
You can also self-host a different frontend or full-stack (non-headless) CMS on a subdomain (or a subfolder/path if you must, with a lot more routing complexity). For flat-file blogs, probably Grav or TinaCMS could work for both you and the user (because they include an editor GUI). Or else Astro is easy to deploy as a dev, much simpler than Next, but it doesn't have a built-in editor GUI so you'd have to pair it with a headless CMS (or make your own simple one).