The Rollei 35 was (still is?) the classic climber's camera - combined with a padded pouch such as those made for it by CCS. It's not just weight, it's also how handy the camera is to use and how well protected it can be while still convenient for use, but not a physical hindrance. You don't want taking a picture to be much of an interruption, or something that you don't feel like doing when it is 3 am, well below freezing and you are not feeling all bright-eyed and bushy-tailed.

I still use a Rollei 35 when climbing (rare these days), an M7 while walking and a weatherproof compact Nikon (35 AWAF) when ski-ing - the AWAF can survive on its own without a case in all weather conditions, and is very fast to use. Before I got my Rollei I carried my Nikomat inside my rucksack when climbing - this is not ideal, not so much because of the weight, but the hassle of getting it out.

If your freind is doing one of the 'normal' routes on Mt Blanc it will be more of a strenuous walk than a climb. The altitude might also cause a small problem - the effects of altitude are highly variable from one individual to another, and nothing much to do with fitness. Here's a snap taken with the Rollei 35 looking down the Dome du Gouter 'normal' from the top of a route on the Italian side of Mt Blanc.

Best,
Helen