Hidden or deleted messages in Facebook messenger.

Hi, I'll get straight to the point. So my partner and her ex have been using messenger as a main communication source for years... which they have been using to discuss matters involving their child. Without going into detail, basically he has all of a sudden somehow deleted all messages from their conversation (dating back years) but you can still see his profile picture and the part where the message's are 'suppose' to be but they are now all just blank spaces, however all my partners messages/replies are all still there... This is very dodgy on his behalf as alot of these messages could be used as evidence in a potential court hearing...

(Just for some context, alot of these messages are of a bullying and harrassment nature.)

My questions are:

Is it possible to get these back? If so how?

How has he done this? I am aware of ways you can delete messages in conversations so that the other party cannot read them anymore but they leave a 'you unsent a message' label where the message was suppose to be and you need to go through each and every message individually to do this... (I have tested this method) so I assume he has used a different method to this?

I would greatly appreciate any help on this matter! Cheers.