When my partner is feeding the baby, her glasses slip down because she's looking down at the baby, and she has both hands full, so she can't push them up. I'm happy to push them back on when I'm there, but I was wondering if there was any advice from bespectacled mothers on how to handle this problem.
I bought some small braiding rubber bands (which you can find in the "ethnic hair" section of any pharmacy next to hair relaxer) and wrapped a couple around the ends of my wife's glasses like this. She says they stay up now.
This type of rubber band is good because it doesn't fall apart after a few months and doesn't grab at hair the way a regular rubber band does.
I know this hardly seems like an answer at all, but better fitting glasses are probably your best option. Mom likely doesn't even need new glasses, but maybe just an adjustment for a better fit around the ears.