I used to have an old CJ7, and I can tell you that a bumper mounted antenna has half the range that the antenna is actually capable of. If he insists on mounting it on the bumper, that is certainly his choice, but he'll get MUCH more range if he mounts it higher. The roof is best by far. Maybe a nice magnet mount that he could stick on there through the top of the rear tailgate. If it's a CJ7 or Wrangler then see if you can find one that mounts to the roll bar. Especially if he offroads in hilly or mountainous areas.