If your mom is used to the glossy iMac screens then why not consider the 24"
Dell S2440L. It uses an AMVA Panel, the colors look rich, especially Blacks and Reds. This monitor is a little reflective but no more than an iMac. Positioning the monitor is key.
The Dell U2412m is a good choice, but will your mom be happy with the anti-glare coating on the screen, some users of the Dell U2412m , Dell U2312hm find the anti-glare coating rather aggressive including myself.
Either way both monitors from Dell have a very good 3 year warranty, if the monitors develop a fault then Dell will send a replacment the very next day.
I previously owned the Dell S2440L and replaced it with a Dell U2312HM. I personally found the S2440L color / vibrancy a little harsh on my eyes. The S2440L is nearer to an iMac or Apple Cinema Display than the Dell U2312hm or U2412m. Obviously the S2440L is not going to be as good as the iMac screen or Apple Cinema display but the Dell S2440L is the closest I have seen yet to both those displays.
Finally, another consideration is the
Asus 23" MX239H IPS, it has an AH-IPS screen non aggressive anti-glare coating and very good reviews.