As galling as it may seem, the insurance company are sticking to the term’s & conditions of the policy. They always check these at the time of a claim where a possible exclusion may apply.

If he’d been mountaineering when the policy was taken out then that was non disclosure & would void the contract. If he took up the hobby years later, then it likely wouldn’t. Life insurance doesn’t tend to get underwritten again at a later date unless you increase the cover.

As an aside, I’ve personally seen an insurer pay out a critical illness claim on a cancelled policy after the life assured had died, when his next if kin found old paperwork. So they will pay out where the T&C’s have been met.

If the honest answer is “I don’t know” then that’s the best answer to give.