She uses Thunderbird email client.
Thunderbird usually generates "User-Agent:" header line. I don't see such header line in the header you quoted. Message-Id also appears empty.
Please tell her to enter the group's email addresses in the To field and your email address in the Cc field, and post (or email me Lena{}lena.kiev.ua) the header of the copy you receive entirely, unredacted.
Lena