It's not just about training-- strategy can make or break a battle. It also helps if you pick up TM's, but that's not required. Just makes things easier, especially since Pokémon like Marowak can learn a pretty wide variety of moves, like Ice Beam.

But I'm biased-- I like to give my team as many moves as possible, but I know that's just one way to battle. I'd say give it a try, and if he gives you a hard time, then worry about training.