Myself, I don't see how the 5x8 DD would be much better than the stock coil. They are the same appx size.
But I guess it all depends on what type of hunting he does, and also the ground..
I don't use any of the DD's on my 250. I have the stock coil, the 9x12 when I want more depth and sweep
coverage, and the sniper for the trashy areas. I think I'd rather have either one of those over the 5x8 DD,
as they should add more of a hunting change vs the stock coil.
But being I don't use the DD, I can't really say exactly how well it works vs the stock 6.5x9 or whatever size
it is. I think it's 6.5x9 if I remember right. Which being slightly larger than the 5x8 DD, in theory it should go
just as deep, or maybe even slightly deeper than the DD.
Everyone seems to think the DD's are really hot on bottle caps, which doesn't really entice me too much. :|
The 9x12 gives a solid 2+ inches over the stock coil, and the sniper is so handy in dense trashy areas, I
almost consider it a "must have" coil for the 250. And it goes fairly deep for a little 4-4.5 inch coil.
Lets you pop, pop, pop the coins out of trashy areas much easier than the larger coils.
But it's his call I guess..