The person who reported it replied by email. Checking the activity log just now (thank you for that suggestion) I see that her post did show up on the group.
She also replied again to the same message thread, and the situation happened again -- where her message went to a non-member (a different email address). The non-member attempted to email back to the list, but of course it didn't go through to the List but did go through to her: "I think you sent this to the wrong person."
I'm quite puzzled. As far as I can tell, this has only happened to her.
Harp List Owner/Moderator