I’m afraid your info is a bit out-of-date. Firstly, most Chromebook are pretty useful even offline these days - most importantly, you can create and edit documents offline, which is the primary use-case for OP’s partner. Second, you don’t need to root a Chromebook any more to install Linux - installing Linux is literally just 4 clicks (Settings > Advanced > Developers > Linux), which would then install Debian in less than 10 minutes.