Unsurprisingly I think, I would urge you to pick a YAMAHA XT600E or Tenere over a bulky DR800. No contest if your looking for a capable reliable long distance bike that will do almost everything well. The DR has the edge on tarmac but the XT beats it everywhere else...

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Triumph Bonneville 800 (2004), Yamaha XT600E (1999), Honda XBR500 (1986).

I love my XT! But Bluesman is right about the speed. I can cruise at 120 Kmph but at 130 it seems unstable. Someone told me it was because of the big ugly fat front fender that the XTs have. It is suppose to do 150 kmph, I know that at 130 it was still pulling hard. 120 is plenty fast enough for me though!

Also parts are much cheaper for the XT and most mechanics can work on them. They have been in Brazil for a long time.

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Every where is walking distance if you have the time!

A bit offtopic perhaps and probably bit long. Sorry.
I am really not an experienced offroad rider, and I have very big respect for people here who really do travel tough places (I am only starting). I did most of my long rides on paved roads, sport/sporty bikes, and by far nothing close to my heroes from this site.

I simply would recommend to your friend - try both bikes and even more that he can get his hands on, and see which one he likes most. I am firm believer in "gut feeling" Then sit down and analyse what it will take him to keep bike running. E.g. - spares, consumables. Then think about 2-up. Then get some factual info on known bugs. And then make choice. I think from what you mentioned XT600 is a better choice. Specially since DR800 is different category - more like Supertenere and AfricaTwin.
But generally. There is obviously "lemon" models of bikes, but generally all Japs quite OK in terms of reliability except known few "parts bins"
My DR800 now at 68000 km, no single part replaced in motor or suspension (except bearings and dust caps - I was just afraid to ride bike with THIS old bearings). No smoke, good compression, not taking oil. I got it second hand, previous owner abused bike but kept engine oil change to 5000 km and valve adjustments to 12000 I think. Rest - he did not touch.
I am really amazed how often people are ignorant to taking basic care of their bikes, and how they make reviews on that experience. I do not think that right principle is to select bike that can be ignored service-wise for longer time. But that's probably because I am mechanic in soul and feel offended every time I see ignorance to piece of machinery that somebody put bit of his life into.
My other bike - VTR1000F is at 48000 km. Only bad thing with motor I had - I crashed in France and broke side cover
My other bike - CB600F (sold) did 35000 km, never missed a bit.
My first real bike VT250F did 17000 with first owner and 18000 with me. Yep, I fixed one thing - painted it
My first bike - Dnepr-boxer (Soviet) would only do 1000 km between valve adjustements needed, but those bike really are piece of .... because of quality of materials and making. Still, I did 5000 on it before giving up.
But "lemons" do exist. Any model, any bike.
As for high-speed stability - my DR "weaved" a lot with me onboard after 150 kmph. All my friends tested it, and had absolutely no weave at any speed. Then I tested their bikes - Pegaso, DR650 - weave. So - turns out it was preload of rear shock. Loose it a bit and most probably bike will stabilize. Not guaranteed, but no harm in trying.

Thanks Bluesman, some wise words there so it is not a long post at all - full of good information.

I don't think you are off topic at all BTW

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Dave