A Facebook archive will provide a file with all of your messages - even between people you are no longer 'friends' with, photos and videos, profile information, etc.
(This guidance is for a computer version of Facebook) .
On your Facebook page, click on the little down arrow that you use to Log Out and click on the Settings link just above Log Out.
This will bring up your 'General Account Settings' page. Click on the link 'Download a copy of your Facebook data' underneath the General Account Settings' box.
This will display the Download Your Information page. Click on the green 'Start My Archive' button.
You will be asked to re-enter your password and confirm your request. Facebook will then compile your data and send you a notification when it is ready for you. Select the notification to display the Download page and click on Download Archive. You will have to enter your password again then your archive will be downloaded to your computer. When the download is complete a new window will pop up with the contents of your archive folder. It will usually be in your Download folder but it is a good idea to store it (or a copy) into a location where you can easily find it or even email it to someone you trust. You can hand this over to your solicitor who in turn will advise the right time to hand a copy to the Police.