How much DOES he wanna shell out? I've seen the flat black older ones for about $1000 before. Usually in .44, but sometimes in .50AE if you keep your eyes peeled.
It really is more of a toy though. I've got one of their ad books, and their biggest selling point it seems was the list of kick ass movies it's been in, that was front and center. It literally lists all the movies in the front. So I think they even view it that way, as a toy.
I thought AMT and Automag, one or the other or both, made one in .50AE, but I could be wrong. Those 80's super mag autos didn't work too well, none of them, and they're hard on parts and hard to get too. They're also collector weapons and they cost more than the DE most likely, which will work better since it was made to handle that round (it has two recoil springs and is gas operated with a rotating bolt like a rifle).
I used to want one as a range toy too, but I got into shooting 10mm and with the Glocks, it's even practical. I have a Smith 1006 for a range toy, it's a serious beast. Not DE, but still.
I do have a .50Beowulf upper for an AR and that uses the same bullets as the DE if I'm not mistaken (I haven't loaded for it in a while) plus a lot of other weights. You could build a pistol lower if you want a pistol, and I've seen 'em before (you'd want a carbine length buffer though I'd imagine) or just a carbine which is more practical really, and depending on where you source the parts and what kind you got, you could probably do it for pretty cheap. They're accurate and they hit about as hard as the .500 S&W (it's kinda like a .500 S&W semi auto). Real accurate to 100-150m, they drop a lot after 200m, in a carbine anyway. They're hell on hogs and it's a serious cannon in an AR package. 11 rounds in a 30rd. mag. You can convert used GI mags to feed pretty well. They come up used often for good deals because unless you load, they're impossible to get ammo for as well as expensive to feed.
Good luck.