Do you use MS Office there? I've pulled some GOOD pranks with autocorrect. Its easy to have it replace a word with something else.
Back in high school, there was a girl that I knew used the same PC as I did for one class, only I had it in the morning and she had it in the afternoon. I setup autocorrect in Word so every time she typed her name, it replaced it with C**T B***H. (We did not get along, and she had given my younger sister problems before). My buddy happened to be sitting in on that class, I guess the reaction on her face was priceless. He came up to me and asked if I had done something to the PC and explained it, I couldn't hardly restrain myself.
It's fairly easy to add stuff, and the non-tech savvy wouldn't know how to fix it. I personally would change it so every time he typed the word "internet" it replaced it with "I know what you did last night." It doesn't have to be "internet" just something you know he'll type at least once in the day. A common word like "the" already has its own autocorrect setting, so I wouldnt do that.