OP, tell your friend thanks for his service.
My understanding from friends and family members who served is that you will normally be issued a multitool, as someone else said. I agree that a multitool is the most useful, all-around item and the ONE thing I'd never want to be without--I'd even choose a multitool over a standalone fixed blade, if I had to choose (see Les Stroud). That said, for a gift, it might be nice to have something a little more "special" and less utilitarian, especially if he's going to get a MT issued anyway. I'd get him a quality fixed blade, probably in the 4" to 6" length. From what I hear, most fighting Marines use guns for fighting

, and that nearly all their knife use is restricted to utility tasks like prying, cutting straps, opening boxes and packages, etc. So a good tough fixed blade is just the ticket, probably don't need a giant blade in the 7" range.
Recommendations? Not knowing your budget, but guessing it based on the Kabar idea, I'll suggest stuff that's (mostly) under $130.
* ESEE knives. Either the 4, or 6, would be a great choice. The 5 is good too, but it's SO heavy. I think for a soldier, the ESEE 4 would be perfect. You can get a full "4" package with sheath and molle locks for around $83 at Knifeworks.
http://www.knifeworks.com/ratcutlerymodel4kydexsheathwithmollelocksplainedge.aspx
* Cold Steel SRK. This is a great, all-purpose soldier's knife. About $107 at Knifeworks, includes an adequate sheath.
http://www.knifeworks.com/coldsteelsrkvg-1sanmaiiii.aspx
* Swamp Rat Ratmandu. This would be my best recommendation. These are VERY hard to get, and one downside is, it'll be $138 for the knife, plus you'll still need to spend around $50 for a sheath. If you can afford this option, this is what I'd get him. If not, then probably ESEE.
http://www.shop.ratknives.com/produ...3999E.qscstrfrnt04?productId=208&categoryId=1
* Fallkniven A1 (6.3" blade), S1 (5.1" blade), or F1 (3.8" blade). These are good stainless knives. More expensive, and the included sheaths suck. But these are truly great knives if they are in the price range. I'd probably get him the S1, it'll cost around $150 plus you'd need a sheath.
http://www.knifeworks.com/fallkniven-1.aspx