Computers aren't designed for high-humidity environments. I would just remove the computer. Or instead, you could bag up the computer.
In any case, it should be turned off. It would be wise to wait about an hour after all the condensation has subsided before using the computer.
The 'fog' would be composed of Nitrogen in a gaseous state.
It would likely stay lower to the floor anyway, so if the computer is on a desk (elevated), there should be no harm.
Turn off the PC and don't let him turn it back on until its completely dried out... It rained so badly inside my house my plasma TV was flooded, let it dry for a week and it was perfectly fine. But this is humidity so this could take a few hours to be safe, I'd take out something of his computer that doesn't turn it back on and tell him that room was very humid and you didn't want to break it, hehe.
GL with the prank!
No, unless the fog has moisture to it, then yes. There is a possibilty.