You could also ask her to send another message to the list and to CC: you on
it at the same time. In the copy you receive, look at the address she's
sending from and see if it's the address she was using when posting worked
for her.
?Also look at the address she replied To to be ?sure it is the correct spelling of the group's posting address. Given that she's replying to a group message I don't know how that could go wrong, but it is easy to check when you get your? CC'd copy of her next reply.