My friend works for the military and wants to play Runescape while deployed overseas. He currently plays OSRS on the Steam Deck via Bolt Launcher.
Is there a way for him to play the game offline at all?
I spent some time researching this because I couldn't find an answer, but the tl;dr is yes, but not officially.
First: None of these methods use Jagex accounts or servers, and I couldn't find a good way for Rs3 or OSRS. These are all remakes of different eras.
For 2001 / 2003, there's OpenRSC. They have a wiki page for hosting your own server that you can follow: https://rsc.vet/wiki/index.php?title=Running_your_own_server
For 2004, there's 2004scape. You can get a copy of the code, and follow the readme (quite short) to get it running: https://github.com/2004Scape/Server?tab=readme-ov-file#getting-started
For 2009, there's 2009scape. Their launcher can be grabbed from Flathub and has a singleplayer option (doesn't require downloading any code): https://flathub.org/apps/org._2009scape.Launcher
For 2012, there's Darkan. You can run this singleplayer and locally, just follow the instructions in the readme: https://github.com/DarkanRS/world-server
yes, but not officially.
... None of these methods use Jagex accounts or servers
I think it's pretty clear that they are unofficial private servers but I do agree that the answer is more like a "no, but here are some similar alternatives to runescape" rather than "yes but with some caveats"
Dr-Bracket provided some good answers, however, being former military who was deployed to Iraq and Afghanistan, I'd like to point something out. The internet out there is not as bad as you might think for games like Runescape. You might still be able to play it multiplayer. After all, I still played World of Warcraft: Wrath of the Lich King when deployed on the internet out there in group heroic dungeons, even with the latency.
Games like those aren't as bad for a connection as you might think. They don't take up much more bandwidth than an IRC chatroom. The real problem is the latency, or the time between you performing an action, the server receiving the your action and you receiving it's response to your action. That time might be quite high. Even still, that can be mitigated if you know what you are doing. For a game like Runescape, I'm sure there are many articles about playing it overseas already.
There is a different issue if there is packet loss. If the internet on the base where you are located has such a bad connection that packets are frequently dropped rather than delivered, your connection could be dropped repeatedly. In this case there isn't much hope for playing, or really doing anything else, on this internet connection.
If you can find out before you deploy what the state of the internet is from someone else stationed there, that would be best. That way you can plan better. I can only provide my own experience.
You cannot play the Official Runescape game offline.
It is an MMORPG. It talks to a remote server in order to function. Without that remote server connection, the game doesn't work.
Dr. Bracket gave an answer on how to play a game like runescape offline. (though as a warning your single player characters cannot be exported into most of the multi-player servers).
The Cthonic One states that you can probably play the game online because, runescape was designed to function in 2000's era internet (high latency, low bandwidth). It first came out when the internet speed was 64 Kbps. And latency was a couple hundred ms. It