Can you use your apple ID on someone elses itunes and what would happen if you did?

Yes, you can. You could buy songs off of iTunes on that person's computer, but they wouldn't sync into yours. If you bought something off iTunes, synced it with your iPod, then hooked up your iPod onto your computer, then it will overwrite all the songs on your iTunes to the ones from the other person's iTunes or vice versa. You can use your Apple ID on someone else's iTunes, but I don't suggest it.