I got my hand built computer stolen this summer and all my electronics, insurance can be a BITCH!!!! however they should totally cover the airsoft, they should cover ANYTHING stolen, regardless of when he purchased it.
However, sometimes they can be dickheads and ask for proof, however, considering he got some electronics stolen he shold have receipts for those and considering he is a major airsofter, hopefully he was proud enough of his weapons to take pictures of them, then he just gots to show that for proof and explain that even though they arent real they are expensive and get an estimate of the real world value of them.
When its all done, hel be god damn glad he got insurance. They will either give him a check to rebuy all his guns, or buy them for him, however once again considering it is hard to do that there is a very likely chance they will just give him a check.
Anyways just saying, hope it all works out for him, now its time to think about getting an alarm system huh?