Definitely agree with Taggerung. Hell, in a basic 30k tactical squad, you can include extra marines at a big discount, to the point that well-kitted 30k army can likely put twice as many bodies on the board as a vanilla C:SM 40k army. It seems to me that 30k is designed to allow a player to field as many marines as possible in order to convey the grand scale of the Horus Heresy.
A 30k list CAN be played against a 40k list, but personally, I would never do it. Tell him to play with a standard GW codex, or find another opponent.
With that said, you can definitely jump on the heresy bandwagon and field your marines as a 30k army (long as your opponents are willing to let it pass, of course). I'm planning out a World Eaters army that I'll use as a 30k or 40k army, depending on my opponent's preference that night.
- April's Featured Fiction winner!
A World Eater survives another day at the office (barely).