Wiki excerpt:
Error EAUTHFAIL 0x80010017
Failure in authorizing SELF. Program authentication fail (Error starting BluRay game).
It's the kind of error you could get when trying to launch a homebrew on a non jailbroken system or when trying to launch a "bad" executable file.
However you cannot lose jailbreak or swap executables just by connecting to the Internet, something else happened, it would be easier for everyone if he could tell you what it was exactly.
What did he install? Ask him, he is better off telling you at this stage.
What kind of setup are you using anyway?
CFW or HEN?
If it's HEN, make sure it's enabled.
Do homebrew other than multiman fail to launch with the same error?
Can you install PKG files? Try reinstalling multiman or test another manager like Managunz..
You could also launch the PS3 Toolset in the browser to find out whether the system is currently jailbroken, if you see "Custom Syscalls: No", it's not.