We can only hope for the best, then. What about the archiving issue: is what screwtape suggested possible? Or would the upgrade have to come first?
It can probably be done without an update.
The forum admins can do pretty much anything within the confines of the forum software, but they don't have access to the files and database that determine how that software runs. In other words, they can drive the car, but they can't check under the hood. That's perfectly fine when the forum's running well, but doesn't give them too many options when something goes wrong.
Backing up from the database is pretty easy; usually the option is present in the interface itself. This is assuming that Mr. Brain and/or his hosting company are doing regular backups of the forum. My guess is that the former is extremely unlikely, and the latter is fairly unlikely.
From the forum side, backup is a bit trickier. I'm not aware of any mods that will do it for you, and even if there are some they're unlikely to be supported. So you're probably looking at a web crawler/archiver, which will likely do a suboptimal job in an excessive amount of time. But it's better than nothing. There are a lot of free ones out there, so it's likely a matter of finding the one that fits their needs.
As for the upgrade, it's probably not all that necessary. The 2.0 release candidates were stable enough for everyday function, and just because the forum is running RC4 doesn't mean it's on the brink of collapse. The real issue is that you can only do minor updates from the forum side--say, from 2.0.1 to 2.0.2. To make a larger update (from RC4 to 2.0), you need access to the actual files.
The main problem is that this forum is in a software limbo. SMF 1.x and SMF 2.x are both supported, but the 2.0 release candidates stopped updating when SMF 2.0 was released. This means that new mods and new updates to old mods will likely only support 1.x or 2.x, but not 2.0 RC4. So the issue is more that the forum is being left to stagnate than it is that the forum will crash in the near future. Sort of like being stuck on Windows ME.