How do I reconnect standalone messenger app to old account?

My mother has been using the standalone messenger app on her phone for a while, and recently I created a Facebook account for her, and the standalone app disconnected from her local old phone 'account', and was forced to connect to the new Facebook one on her standalone phone app (it 'logged her out').

So now all her old 'phone based' standalone history is gone, and she wants it back - reconnect to it, as it has important messages from people she is not 'friends with' on her Facebook...

How do I reconnect standalone messenger app to old account?

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