I've got a friend who's having some trouble with a friend she's got stuck in the past, and it'd be kind of bad if he found out anything about what's waiting for him in the future. I think her plan's something along the lines of "don't tell him anything", but you know how it is, there's always someone there who might mess it up.

... Basically, what are the rules with trying to pull the strings on time travel stuff without causing a time paradox of some sort, I guess.