Bummer that nothing has helped clear up the issue, and hopefully it does go away with something like an update to the Chrome app. Her Nord 2 is about a year old isn't it? OnePlus should still be providing monthly updates (or bi-monthly) yes? Perhaps that might fix things.
Is this something that is really bothering her every time she sees it or just some minor irritation that's mostly just a curious observation? It's just an icon in the Notifications, like most of us, in due time eventually she'll just get used to that grey box being the Chrome app notification (...and probably be surprised when/if it does return to its original colored ball-shaped icon).
If those generic, grey icons in Notifications are something she's for whatever reason fixated on, this is for the most part of a tentative suggestion but switch her phone to use a different web browser app. Instead of Chrome, there are numerous alternative browser apps available. Chrome is a very solid performer and it's frequently updated to keep up on online security matters, but it fails as far as privacy. There a several Google services on our Android phones that are constantly data mining us, the Chrome browser app being one of them. Switching to third-party web browser app will reduce at least some of Google's oversight on her web browsing.
-- There are plenty of viable alternatives to choose from, I prefer Mozilla's Firefox Focus, but there's also very popular alternatives like Brave, Microsoft Edge, Opera, and many others. If she's tied into the Chrome environment (i.e. her phone's Chrome browser app is synced with her Chrome browser on a computer or whatever) that's going to make things messier but if she just uses the Chrome app to do typical web browsing that makes it easier to switch.
-- Once you've picked an alternative browser app and installed it on her phone, go into the Settings >> Apps >> Default apps menu and select your new choice as the Default browser. Now go into the Settings >> Apps >> App management menu. Find and open the Chrome app entry and tap on Disable. (It's installed as a system-level app so you can't readily Uninstall it, but at least if you Disable the app it won't be active in the background using up system resources).