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Hi Matty - the earlier bikes (2008) production sometimes had a problem with excess oil being blown through the crank breather into the airbox, then collecting in the drain tube (the tube should have a bung in it so it doesn't leak out) - this tended to happen when bike was used at high speeds on a constant throttle (ie Motorway speeds), and was all but rectified for the 2009 model year with a revised crankcase breather box (ie. all OZ spec bikes should have the newer shape box). However, it can still happen if used for long periods at high speed/revs.
The actual amount of oil collecting in the tube ought to be minimal though (if you empty it into bottle you'll see how little it is) so only thing I can think of is that perhaps you mate has put too much oil in (if he feels the level has been up and down and been topping it up) having not checked it at the correct temperature for example, and so it is blowing the excess out of the breather?
http://www.advrider.com/forums/showthread.php?t=265147&page=223
Overfilling won't have helped, but you still wind up with a bit of oil in the airbox anyway. It's a big single, there's a massive change in crankcase volume when the piston goes up and down, so there's a massive change in pressure.
Is your bike a 2008? 2009-onward models had a slightly different design of breather chamber which was supposed to separate the oil out from the air a bit better. Yamaha were changing them on the 2008 bikes for free, mine got done at the 12k service - do you know if yours has been done?
The thing to do is to keep an eye on it and drain it regularly by taking the bung out of the drain pipe. If your bike is losing loads, you might have cause for concern. If it's just that noone's drained it since your bike was new, then it's not surprising it's got loads in there.
http://www.adventurebikerider.com/forum/27-technical/83119-oil-in-the-airbox-chamber-on-xt660z-tenere.html