Use an amp that can provide around 500-750 W RMS (can handle more in the right box, but thats a whole other convo)
If he has dual 4 ohm voice coil subs, then the load will be either 1(most common way to configure), 4, or 16 ohms to the amp. If he has dual 2 ohm voice coil subs, then the load will be either .5, 2(most common way to configure) or 8 ohms.
Look for an amp that provides the power in either the 1 or 2 ohm loads (depends on the sub configuration)