Capcom released a small patch for the Steam version of Resident Evil 5 that adds local split-screen co-op. This will allow players to play with a friend without both needing their own computer or copy of the game. There is also no longer support for Games for Windows Live on Resident Evil 5. [Thanks, Wario64!]
Local split-screen co-op is quite literally what it says on the title. So long as there are two controllers for both players to play, you will be able to co-op with a friend on the same system. The screen will be smaller as a result, however. This was the method of offline co-op that Resident Evil implemented in the past, such as in the original console iteration of Resident Evil 5.
As for Games for Windows Live, this was a gaming platform that originally came out during the Xbox 360 era. While Windows no longer offered official support, some publishers still did. However, that will no longer be in the case for Resident Evil 5. Players who still used Games for Windows Live for Resident Evil 5 will no longer be able to do so after this patch.
Resident Evil 5 is available on the PS4, Xbox One, Nintendo Switch, and Windows PC via Steam. As of this patch, PC players will also be able to play in offline split-screen co-op. The same patch has also fixed several minor bugs and issues.