I'm not going to guess about the prices, since they vary so wildly, but what kind of work does she do? multiple "small" monitors might not be the best deal compared to some "larger" high resolution ones depending on type of tasks.
(and those tiny monitors very often have low resolutions on the really cheap ones, which can be counterproductive in itself)
That ASUS monitor looks pretty decent for that price. No DP port, but with a MacBook 2016 you probably already have more dongles than a gadget store, so shouldn't matter.
edit: just incase you're unaware (and disclaimer about me now knowing Mac very well), you can use the MacBook screen as one screen, and the standalone screen as another (at least you can on regular PCs), so you already have two screens with one external, we have a lot of those at work, people will keep their e-mail client (or whatever info they're looking up on) on the small laptop screen, whilst working on the bigger screen.
it's not what you asked for, and slighly above the price you suggested,but we have several cousins of this one
https://smile.amazon.com/BenQ-GW2765...dp/B00KYCSRSG/ at the office, works pretty well (except for slighly buggy when used with the newer Surface Pro docks)