This is called force Force Majeure (often referred to as "Acts of God") and is excluded in the company's booking conditions and therefore your customer will have to pay for these. This would usually be covered by wintersports insurance, one of the many reasons why people should have a good quality travel or Wintersports policy.
If you wish, insurers will accept a block claim and for you to pay all the bills for those who are insured with MPI. But before incurring any costs please phone us on +44 1428 664 256. For the remainder your company will have their own procedure on how they wish this to be handled, so please refer to your company's own Reps guide.