Not certain but I think this is a very likely culprit. She likely needs to unregister RCS "chat" messages on her old phone, but if it's inaccessible, she can do it from a web page here:

https://messages.google.com/disable-chat

That's Google's page with instructions to do it from both the old phone or the website linked within. The kicker is that they say it can take several days, up to a week.

Think of RCS as Google's iMessage, accept it's supposed to be a universal standard...though Apple refuses to use it as a standard until forced to, I think when 6G becomes a thing. Until then Apple wants this shit to happen so people stick with iPhones.