Something nobody has brought up yet. Did everyone that was flying have a spotter? my club requires one if more than one pilot is in the air at a time. If no spotter were used, you take your chances.
Are you going to get paid becauce of a lost model, most likely not.
Can you sue him in a court of law, sure. bet you will have the whole sky to yourself after that and not to many friends.
These things happen, thats all there is to it.
Time to move on and take this as a lesson learned. When this guy is flying, Land or fly high.
Have the club officers make sure to tell everyone in whatever form they choose to observe the flight pattern when more than one aircraft is in the air.