Thanks everyone for your helpful responses! Here's the update:
I got home after my last post, spoke to my mom again and just tried to emphasize over and over how important this was--that baby was being overfed and that she couldn't possibly need that much milk. I asked her to try other comfort methods as much as possible, and to use the slowest flow nipples (we use the Playtex dropins bottles, which were recommended by my LC).
My mom claims she didn't really do anything differently the next day, but that Zora just chose to drink only 8-10 ounces.
THe week after that I had a vacation, so I nursed on cue all week and didn't pump once (LOVED IT). I found that I was HORRIBLY and PAINFULLY engorged ALL the time, no matter how frequently baby nursed. which leads me to think that I had way overstimulated my supply by pumping that much at work every day!
And now I'm back at work and pumping again... and baby is only drinking 8-12 oz per day and is making up for it by feeding more frequently in the evening and during the night. Again, my mother claims she is not doing anything differently than before, so I have no idea what is going on, but I hope it continues. I'll monitor the situation in case it changes again.
Oh, and just to answer some of the questions--my mother never props the bottles with her. She tries to imitate breastfeeding as much as possible and always holds and cuddles Zora while she feeds her.