...when she tries to reply to someone elses, it won't let her send it.

The only obvious answer is that when she tries to reply, the reply is being sent from a non - registered email address.

Q1: Is she receiving emails from Groups.io on her new address?

Q2: Does she get an error message from Groups.io when she tries to reply?

Q3: If the answer to Q2 is yes, what does the error message say?

A pedantic point, perhaps, but Groups.io does not "won't let her send it". The message may not turn up in your group but that is not the same thing!

Q4: Have you looked at your Activity Log to see what is reported there?

Q5: If yes, what does it say?

       If no. then please try that and report back here.

Ask her to check the address from which her reply emails are being sent just to verify that they are being sent from the registered address. If she is sending from an incorrect address then something is happening (or not happening!) at her end.

Chris