Seems to me the client has more money than common sense if he wants to use his .7 Rem Mag mainly because "he feels more comfortable with it".
I would buy a .338 or .416 and learn to be "comfortable with it", rather than risk wounding something as dangerous as a big brownie. I think it is possible the bullet could break up on its upper leg bone.
I might argue the opposite. A couple of experienced folks have chimed in here stating they'd rather have the client with a gun/chambering he/she is comfortable and familiar with. To me, the guy wanting to use his 7mm mag shows he's a somewhat rationale and reasoning human being. There's no way of knowing if it's a financial based decision, or perhaps recoil tolerance or familiarity are the driver. Would it be my pick? Nope - but it'll work.
The guys with more money than sense buy a 375 or .416 a month or two before the hunt. And it's all relative anyway - some of those folks will put down 15-25k on a hunt and beotch about the price of the factory ammo so they won't shoot it much. Or, if they do shoot it a bit, decide they've bitten off more than they can chew but will use it because they have to have it based on what they've read.
When I went on my first brown bear hunt, I didn't even bother posting about the anticipation leading up to the hunt because I knew there would be 6 pages of "there's no way I'd go with less than XXX chambering and I'd be shooting XX bullet if it were me". And 90% of the responses would have been from folks that had never been.
I used a 35 Whelen and anchored my bear on the spot and put a few more in him for good measure. The two hunters after me (in speaking with the guide a couple of weeks later) used a 375 Ruger and the other a 416 of some flavor. Both of those hunts led to a rodeo in the alders afterward - one bear was never located.
The particular outfitter/guide knows rifles/bullets quite well and shares Phil's mindset when it comes to hunters and rifles.