Which 27" monitor for mac mini at lower resolution to look larger? I'm considering getting an HP Z27 (4K UHD) or HP Z27n G2 (QHD) for my mother.  She has a (2018?) mac mini.  I plan to turn it to a lower resolution so that everything looks larger, because it would help her to see.

If the resolution will be lower, is there any point UHD, or is QHD just fine?  Or does even QHD not work well if I turn the resolution down?
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.