Let's back up a bit. You said "her email to our group bounced" which sounds like she got an error message when she tried to send a post or reply to groups.io. To help, we need to know what error message that user got from her email service.
The "most recent unsuccessful attempt" is a record of last time there was a failure when groups.io sent an email to the subscriber. When was the "most recent successful delivery" to this user? If successful deliveries are reported after the most recent unsuccessful attempt, then she is receiving email from groups.io. (meaning whatever caused the earlier failure is no longer blocking email)