Is the 12k canadian? Motorcycle Consumer News has the 2012 800XC Priced at $9865. At $12k, your friends bike should have a TON of upgrades. They are often on Craigslist in that range.
http://seattle.craigslist.org/sno/mcy/4168894686.html is a 2013 with 1600 miles with goodies asking $11,250. The US bike sells for $12k brand new.
If you buy the used Triumph at a fair price, then ride it all summer, you can probably sell it for nearly what you bought it for.
If you buy the new Terra, farkle it, ride it all summer, you'll take a big hit if you find you don't like it.
The Tiger has an incredibly smooth road bike engine. It reviews well as a road bike, although the plain 800 seems to be preferred for that purpose. I demoed the XC recently, on pavement, and it's a really fun bike and the engine is incredibly smooth with a lot of power.
The Tiger has ABS, but MCN measured 60-0 stopping distance at 129 vs 122 for the Terra. The Tiger is about 1/2 sec faster 0-60.
They both get about 45mpg or so, although the Terra mileage varies widely among Terra riders. They both run lean, the Terra is an easy fix with a booster plug, wuka plug, etc. The Tiger has an extra 1.43 gallons of fuel. Range on the Terra is 160+, Tiger about 225.
The Tiger is an EXTRA 100 POUNDS. It carries it's weight very high. The Terra is 20% lighter, and feels much more lighter do to where the weight is. Put them side by side, the Tiger dwarfs the Terra. Ride them back to back in slow speed stuff, I can't emphasize enough how heavy the Tiger rides.
The Tiger is more complicated. If it's reliable, that's not a big deal other than greater maintenance cost.
Have you ridden either? It sounds like your going to be spending most of your time off road. The Terra will be much, much better there. On the road, it's the Triumph. Two up goes to the Triumph unless you both weigh 100lbs. If the two up is really occasional, I'd remove it from the equation completely. Rent an HD or a Wing for the occasional two up ride, you'll both be happier.
Disclaimer: I've put 6000 miles on a Strada, 4000 miles on a Terra, they are great bikes. I've put 50 miles on the 800XC across a couple of demos, I didn't look at the Triumph when I bought the Huskys. If I was planning to ride MOSTLY off road as you describe, I would but the Terra again.