If your mother is an English speaker who has lived here 50 years she might already be a member (I say this because 'back in the day', ZIWA was one of the few opportunities for international women to connect with one another) - but if not, she might be interested in the Zürich International Women's Association: www.ziwa.com
While this is not a group for seniors - ZIWA members are all ages - there are nonetheless many women who are older, have lived in Switzerland for decades, who might share your mother's interests.
ZIWA's activities are quite varied: book clubs, music, art, discussion groups, bridge, food and cooking, hiking and other sports, travel, coffee mornings, etc. There's something for everyone, and your mother could be as involved (or not) as she wished.
The language of the club is English, and most activities center around Zürich city.
Since you are in the US - is your mother an American? If so, there is also the American Women's Club of Zürich. Again, not for seniors only, the membership covers a wide age range - but she will likely find friends with common interests. In addition to their various activities, AWCZ also has a library of English language books - your mother might enjoy an afternoon browsing...
The AWCZ clubhouse is in the neighborhood of Bahnhof Selnau: http://awczurich.org
Another resource is the Senior University of UniZH http://www.seniorenuni.uzh.ch/index.html
This a program of courses (non-diploma) and lectures for anyone over 60. While most of the program is indeed in German, occasionally some courses/lectures are offered in English. For instance, this coming up: http://www.seniorenuni.uzh.ch/servic...meetswest.html
Auditing a class is a wonderful way to connect with others who share your interests - your mother might meet some very enjoyable people this way.
What town is your mother in? If you let us know, we could point to some specific resources in her area...