Assuming legal size and weight, hand luggage allowance is part of the contract between the passenger and the airline. In fact SWISS advertises 2 pieces of hand luggage in C, so being denied just one is even worse.
Being right is nice but getting your right honour is something else altogether. I know there are people on FT that would ask them for an IDB voucher. I would definitely contact SWISS (or Edelweiss, depending on who your wife booked with) and ask for reasonable compensation.
Was the luggage rejected by the gate agents or the cabin crew? Unfortunately the gate staff in AMS (at least those handling Edelweiss/SWISS/LH) recently have seemingly become more incompetent - I've had several incredibly inefficient boarding procedures over the last few weeks and they seem to put gate agents there on their first day in the job.