So a friend of mine asked me to help him with his dad's old classical guitar, on which apparently he's had steel strings for years.

Now, he claims the neck is fine, which remains to be seen, but if indeed the guitar turns out to be playable, will I warp the neck (or otherwise cause further damage) if I replace the strings with nylon ones? Seeing as how the neck has probably adjusted to steel string tension and all?
Dear God, do you actually answer prayers?

Yes, but only in a way indistinguishable from random luck or the result of your own efforts.
well the neck will warp without the steel strings on, but chances are that having the steel strings on for so long the damage is already done.

It could still be playable, but if worst comes to worst you'll have to find a decent luthier and get a new neck for it.