I hate cameras on. I find it hard to focus by looking at the people and much easier when doodling, for example. Also, there is usually a lot of screen sharing, so pointless to have camera on for those. I also hate the hair and makeup situation as am not someone who usually wears makeup. I prefer to work from home how I feel comfortable and a video call makes me have to think about how I look, which seems a waste of time.
There is also the environmental aspect - video calls use way more battery and a lot of companies have policies around using cameras to keep their sustainability policies in order. The younger generation (hopefully!) feel strongly about wasting the world's resources as much as the rest of us.
I also have to to rearrange my desk for a video call - usually I have a screen in front of me and either a separate keyboard or the laptop. With no separate camera, I have to move the laptop to the other corner of the desk, and put the laptop on a riser, where I then can't easily reach the keyboard if needed (otherwise you would just see up my nostrils. I find a real mix of people's desk set ups and it can make a difference. I don't see the point where people have a camera on, but you are only seeing the side of their head while they are looking at the big screen in front of them. I know some companies pay for separate webcams and other equipment. Perhaps it is worth asking if she has a desk set up that works for video?
Some tips:
Client/external people call - camera on
Team call - camera on at start and end, especially if screen sharing for the most part. Or, as others have said, if you have a regular weekly or monthly call that she knows are coming, then cameras on, 5 mins team chat at the start, then acceptable to turn camera off
Slack catch ups - nah, no camera required. It's a chat or screen sharing usually and not may people want to be caught out looking a certain way for something ad hoc