Lent SIM merged Whatsapp accounts - how to separate

Hi all,

I recently lent someone I know an old but active card, not knowing that it still had an active whatsapp account associated with the number.

This person restored the whatsapp messages to his phone, and has since changed the number associated with the account on his phone to a new number (for his new SIM). I have got the old SIM back.

The whatsapp account on his phone now has some of my old messages, and his current ones, on the same account.

Is there any way to separate these contacts/messages to their original/appropriate whatsapp accounts?

Thanks.