About 30 seconds later, I'm not sure why, but I turned back around to look at her. The ramp was unfolded and she was trying to roll up it to the bus. But the ramp was pretty steep, and she was struggling. She tried once, and then twice, to roll up. On her third try, I watched horrified as the back wheels of the chair made it to the ramp, and then the entire chair tipped backwards.
I found myself glad once again that I seem to be immune to the bystander effect. I ripped off my headphones and ran back to the bus. Thankfully, the woman seemed OK. I asked her if she was OK, and then if I could help her up. (I'm not sure where I learned this, but I always ask people, even in situations like this where the task to be done is obvious, if I can help them or touch them. It's a dignity thing.) As I started to help her up, I asked if she could stand, and thankfully she said yes. By this time another gentleman had gotten off the half-full bus, and he and I pulled her up. I asked her if she had hit her head, and she said she hadn't. The driver was off the bus too at this point, and he held the chair while she sat down in it. Then he (without asking) started to push her up the ramp and onto the bus. At this point I figured there wasn't more I could do, and calling 911 didn't seem warranted either, so I left.
I walked down the block, and as I turned the corner a minute later, I noticed that the bus was still sitting at the stop for some reason.