I am also a former Navy nuke, and I also had a 99 on the ASVAB, as well as acing the "nuke" test (they give you an 80 question exam before allowing you to enlist into the nuke program - or at least they did, my information is from 20 years ago).
A lot that has already been said is true - higher than normal suicide attempts (though I've not heard of any that were successful - many were just "get the hell out of this shit" half-ass attempts), very high attrition rate, and a 6 year obligation if you make it through the training program (you sign a 4 year contract when enlisting, and extend for 2 years when you finish the pipeline). (Note that you get a pretty decent bonus when enlisting - I don't know what it is now, but it was $6k when I enlisted in 1993).
Expanding on the high attrition, the enlisted nuclear training pipeline a REALLY tough training program. It's the toughest thing I've done in my life... until I did it again through the officer side, which was even harder. The amount of knowledge you have to learn in the relatively short time period is huge. After you finish, you'll just sit back amazed at how much crap you managed to shove into your brain. Many smart people failed, and would go out to the fleet without a job profession (called a "rating", or MOS in other services) if they failed before completing A-school, or as a "conventional" of that rating if they failed out of Nuclear Power School or Prototype. Additionally, there's a lot of people that got caught drinking alcohol underage, or supplying alcohol to underage sailors if they were over 21, and would go to Captain's Mast (a mini-trial where the Captain is judge and jury) and would get washed out of the program and usually demoted as well.
And, honestly, there's a lot of assholes that go nuke. It's a pretty toxic environment. You'd think having a bunch of smart people would make it better, but now you just have smart people under a lot of stress being shit on by bitter superiors and get turned into assholes.
All that said, if you can make it through this program, everything after that in your life is easy. I've not encountered a single professional job/role/task since the Navy that was challenging - as such, I stand out wherever I work, and am quickly promoted. I got a bachelors degree in Materials Science and Engineering and aced everything while watching my classmates struggle. My Master's in Comp Sci was honestly a complete joke after I got out of the Navy, and I went into software development and despite having basically no experience, I was quickly promoted and assumed tech lead and architect responsibilities. Learning a hugely complex and integrated system like a nuclear reactor makes it much easier and less intimidating to dive into complex systems elsewhere and learn them quickly - like say a legacy huge enterprise software system.
So, if he just wants to get the awesome GI Bill benefits and get paid for a few years while learning a few interesting things, but not having to deal with a huge amount of crap or have to work too hard, the Air Force is a great place to do that in - my sister did exactly that in the AF, and came out with great benefits, but hasn't had as much success professionally as I have.
If he wants to be challenged as he likely never will be again in his life,and have an uncomfortably high chance of failing, and be set up to blow everyone away after he gets out if he succeeds, then nuke would be a good way to go. Or to put it another way, this path is the "crawl through a river of shit and hope you come out clean on the other side" - I'll reiterate that it REALLY sucks going nuke, but the benefits on the flip side can be pretty substantial.
