In fact, if her needs beyond writing are limited (eg no need to play games or run some proprietary software), then a Chromebook would probably be the best option. One thing I’d recommend though is to go for an ARM based Chromebook (eg using Snapdragon or a Mediatek chipset), as these tend to have a much better battery life compared to something with an Intel or AMD CPU. ARM Chromebooks also tend to be thinner and run cooler. Also, ideally I’d recommend getting at least 8GB RAM for better multitasking.
The problem with iPads on the other hand is that they can be quite expensive, and lack some of the flexibility a Chromebook might have (eg, say you wanna store your written documents locally and then copy it to a USB drive or something). Most importantly, keyboard covers are kinda meh - especially for a writer - and the lack of a proper hinge/support means you can only use your iPad whilst it’s propped up on a table, so you can’t use it say if you’re lying on the bed or couch etc.