Every where I have read in the manuals & specifications has been in the 450 ft-lb range.
I would question this friend as to why he thinks they need to be so tight. Over tightening causes stresses the rims, hubs, & studs were not designed for. This can warp hubs, pull out/ break/ strip the studs, cause the rim center to crack out, mash the metal out from under the lug nuts, .......
Now if he is trying to generate more service business, that would be one explaination why.
It is so hard to find knowledgable service help when you know what is involved, almost impossible when you don't.
Most of the tire stores I've been in do not understand proper torque. They believe more is better! Heck, most don't care!
It is your coach, buy the manual (if you don't have one) and know the requirements for proper maintence. You will save your $ for more fun stuff.
Good luck!
kyle4501