Fitter:
is your friend receiving treatment from a psychiatrist or a psychologist?
Of course not. He's British.
He's not at a dangerous level yet I think. But I do recognise the territory he's in. He's living entirely in his own head. He is his thoughts. And that fundamental issue is as you know, a vicious circle that will eventually express itself in the body as well.
Some Guy:
My take is, any improvement in present moment awareness, self-awareness, awareness of one's own reactivity, is a beginning in basic coping skills that a lot of people just don't have. When I see people looking for answers to stress, I recommend anything that features mindfulness (especially of the breath) that is something they want to do. It could be yoga, tai chi, etc.
Yes. I would send him to yoga class if it weren't for the shocking state of what laughingly passes for yoga in the UK

Im a pretty experienced (though again unqualified) yogi. If I go the breath route, I would like to incorporate some yoga for when he has time.
For example:
20 - 30mins light yoga practice. Just enough to mobilise the joints, stretch the major muscles, help improve posture and all the while
connect the body with the breath. (i have a great routine for this that I do myself often)
10-15mins systematic relaxation though body scan meditation.
20 - 30mins mindfulness of breathing.
When he has time. Do the whole thing in an hour. When not, skip right to the breath and sit for 20mins.
Eric:
I think your leaning toward the first option is right. The directions are simple and nearly impossible to forget or "misremember", as is employing the actual technique. No books necessary. I think a straight up samatha technique is what will give you the bang for the buck (or quid) you're looking for.
The point about simplicity is well made. And on this note, it may be worth putting aside yoga and body scan until a practice is established. With Anapana there's nowhere to hide. Nothing to procrastinate over. He can either do the practice, or not.
I am not keen on mantra at all. He actually went to a TM open evening. Once I realised what it was, I told him to run! Though i do take the point re concentration, he'd probably see quick results --but then this does not provide a basis for further practice should he wish it.