the 70-300 is worth a look especially for travel but I would vote a 17-40/16-35 + 40 STM or other small prime + 70-XXX and maybe a tele converter as an ideal travel setup. If its a once in a lifetime type thing I would put no restrictions on gear.
I agree, he has the 16-35 already, so add in a light/compact prime and the 70-300L (in my opinion Canon's best tele-zoom for travel period) and he is set. Should all fit in a very compact bag as well. Light weight (comparitively), maxiumum versatility, and not attract a bunch of attention.
At the current Adorama price of $1,250, it will save a lot of coin too.

