The two HP 27" models that you mention in your question are the very same two that The Wirecutter recommends in its review of the best 27-inch monitors. According to the article, current (Sept 2019) pricing is US$540 for the 4K/UHD/2160p version, and US$340 for the 2.5K/QHD/1440p version. Based on your description of the intended use for the display, I'd say QHD resolution will be just fine.
If your mother would be satisfied with a 24" display, you can save some money by buying a display of that size. The Wirecutter's review of the best 24-inch monitors recommends the Dell UltraSharp U2415 (currently US$225) for QHD/1440p resolution, and the Asus VS239H-P (US$130) for FHD/1080p resolution. At the 24" size, either resolution should work fine for your intended purpose.