Is your buddies box designed by JL for those subs? If so it is very hard to make the box better. Unless the box is poorly constructed. But assuming the enclosure is properly constructed and it is a JL designed enclosure not likely to get better. Now if the box is not properly constructed build a new one that is. If it is not a JL designed enclosure find out what JL recommends and possibly build another to their specs.
Or he can add two more woofers or double the power.