I have checked the Activity log again. There is nothing from thisI don't know what to tell you. Those two facts don't go together. There must be something we've missed.
For example, if she was posting from an address which is being logged, but which you don't recognize as being her.
See if you can get her to try once more, and when her message "bounces back" forward that bounce message on to you. The bounce message should have a sentence that explains why her message is being sent back.
She has given up trying and I think I have too. Why does everythingIt shouldn't be, and I don't think it is for most people most of the time. But that's no comfort for you or your member.