Most messaging apps are centralized. Some form of server side checking can be easily added to make sure the client is legit and not tampered with. Same form of checking thing if it's P2P but it would be very cumbersome.

Now it is possible that someones has cracked the encrypted messaged payload and figured out how to fool the client authenticity checking part but at that point you are probably dealing with a state actor and even apple with their secure app Store cannot save you.

That's not state actor level of effort. Kids do that stuff for video games.

Why do you care what software your friend runs? Why do you feel the need to dictate what software someone else runs?

This is especially weird because your username is 0xEFF which initially made me think Electronic Frontier Foundation but it's definitely not that.

I don’t feel the need to dictate anything, but I care because my messaging app exchanges my personal data with my friends messaging app so there’s value knowing their copy is authentic.

Your friend's messaging app is not installing apps on your phone, so it doesn't matter (to you).