I too am sorry for your sister and brother-in-laws lose. Very good thing there was no one in there.
Unless he had a rider on his home owners insurance policy, would it really cover an avalanche?
I know a standard insurance policy does not cover earthquakes, floods, mud slides, etc. I believe you have to pay quite a bit extra for these type of "Act of God" events coverage.
Maybe since it sounds like a mudslide came through there once already, that he has all the necessary insurance in place. Heck, maybe with a place like that for a "cabin" he is well off enough that it won't be an issue either way.
Good luck with the outcome.