I told a breastfeeding mom she was leaking
What was your reason for telling her that she was leaking?
Because it seemed like the right thing to do
It is embarrassing to point out but it is good that you let her know. Now she could fix it so that she won't be embarrassed when someone else notices.
I didn't even know her though.
Well it was still nice to point this out. Hopefully you did it in a way that did not draw too much attention to the situation. This would have saved her the embarrassment.