It wasn't so much exposing her as being offended; she was put out by the treatment, but had gotten used to it over time. To the extent it didn't bother her any more (the more I tell this anecdote, the more awful it seems really!). When I raised it as an issue, publicly, it brought all of that back.
In addition it made her feel that everyone saw her as being targeted, and unable to cope with it (partly because we were so close, she imagined they saw her getting me to stand up for her where she could not). Whereas before she could think of it as a private upset between her and "Chris".
I never even thought of it in that context; "Chris" was doing something inappropriate, I felt that calling him on it would be a good way to resolve it for everyone.
What I missed is that it could affect "Amy" in unexpected ways. A private conversation with "Chris" would probably have been better...
So, yes, she blamed me. And herself. But only really in the heat of the moment - after the fact she was grateful, just wished I'd done it another way.
One thing she did say was that her initial reaction was that I was just as bad (sexist) because it looked like I saw her as unable to stand up for herself (which wasn't what my thought process was, I should add!). But after she realised that actually she hadn't stood up for herself on this issue. That blurs the line so much on terms like sexism.
So I learned; human social interaction is blooming complicated!