That is horrible news, sorry to hear it. It won't be easy for them to sell Triumph parts without altering them and they will never be able to sell the whole bikes, too noticable. Hope they get caught.
Insurance: The bike insurance per bike for comprehensive pays them. I don't think your insurance will pay for the Big Dog though. Insurance is specific to ownership. Your friend should call his carrier unless you have a commercial policy covering all in your custody. Personal property like the computers etc. comes from homeowners insurance.
This really sucks, hope they get the bikes back. Maybe they were just wanting to steal the trailer and the bikes will make them nervous so they will abandon them.