I don't see what is the problem to delete user's messages at their request. Just replace its text with "message deleted" for example. It doesn't require anythin

I don't see what is the problem to delete user's messages at their request. Just replace its text with "message deleted" for example. It doesn't require anything advanced. It seems that your co-founder just doesn't like GDPR and users' rights. It is a political, not a technical problem. If someone will post something that violates US laws (for example, terrorism-related content or child porn), will you refuse to delete it too?