The Sony Rx10iv is a lovely camera, not terribly heavy, with a 24-600 zoom. When I was there a couple of years ago, I found I used the 100-400 zoom on my Canon more than any other lens, and I'd have loved the extra 200mm.
I expect it would be fine in the cold; she'd be well advised to take a spare battery. I don't know about its weather sealing, though.
Jeremy