Besides ublock origin like the other posted stated your biggest scam threats would be coming from social media websites, Facebook or meta whatever its called and email being the biggest. Since I assume you can’t monitor her 24/7 and she’s an active user, setting up some kind of DNS solution and firewall on your network, you managing her passwords and 2fa via hardware, setting up a non admin local account for windows and maybe some kind of easy reference material like pictures with text in a booklet should be used also. Assuming she’s still sound of mind

Edit: as for the phone portion of the question it really depends on how much media do you want her exposed to? You can make a pretty dumb phone via grapheneos and install the games she plays normally (network access restricted of course) with automatic updates. But if she uses social media, messaging apps and other apps like that maybe an iPhone would be easier and simpler for her. At that point you will have to rely on your network solutions, you managing her account credentials, bank included, and the reference material to protect her from scams