I'm sure many brown bears have been killed in days of yore with a 30-30, but all I can say is I hope you "use enough gun" as a backup.
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.
Perhaps he should go with solids (if they make them for a .7mm Rem Mag).