Is there any way to save all text messages from an individual contact in Google Messenger?

My Fiancee's brother passed away a month ago and wants to save all of the texts between them. She's using Google Messenger currently on a Samsung Galaxy S21 and has backup enabled, but wants to be extra sure the texts and pictures are saved and safe from deletion.

I've looked on the Play Store, but just wanted to ask if anyone has any experience with this or any suggestions so that she can relax and know they are saved. It would be nice if she could select just her brother's conversation.

Thank you in advance for any help!