Merging old and new phone Whatsapp Chat logs

Hello,
I have a question for a friend's issue. My friend needs to retrieve some very important contact info contained on his old phone's WhatsApp.

WhatsApp is enabled on his current phone containing his sim card, which means that he is unable to access the WhatsApp chat logs on his old phone.

I gather that the solution would be to remove his sim card and temporarily reinsert it into his old phone in order to re-activate his WhatsApp on the old phone in order to access the records.

However, my friend wants to avoid losing his new phone's WhatsApp records and is afraid that by reactivating his old phone's WhatsApp in order to access the logs he risk losing his new phone's WhatsApp chat logs.

Could you please advise us on the risks involved and the best way forward?

I was thinking it might be advisable for my friend to make an external backup of his new phone's WhatsApp data, before moving his sim card back to the old phone to retrieve his old phone's WhatsApp chat logs.

Thank you very much!