If i devolve my opponents pokemon, are they allowed to evolve it back on their turn? I dont think they can, but i was at a prerelease and the TO said that you could. So can you?
Yes, as long as they don't happen on the same person's turn there is no question about it. Here's an example from the old Compendium:
== DEVOLUTION BEAM (Mew - Promo) Q. Can you evolve a Pokémon right after Mew devolves it or do you have to wait a turn?
A. If you devolve your own, you just did an attack and your turn is over. If you devolved the opponent's Pokémon, he can evolve it on his turn. (2-10-00, 6-1-00 Q54 & 6-30-00 WotC Chat)