Usually people do report others due to personal interest. I hope its not the case here. But if I were you, I'll just mind my own business, and I'll make sure that I'm a Law abiding expat. Its not your task to report anyone. Let the Authority do their job, I'm sure they're aware of those having expiring visas, if they wanted to catch him now they can do it without needing your help, they have all complete datas in their system. Just hope that you won't get in the same trouble as this guy.