Hopefully some others here can give you a more up to date indication of laws, but I don't think you can just leave your British registered bike there - you will be required to take it with you, failing which it'll need to go on American plates.
Another pitfall is the insurance is probably going to be very high, difficult to obtain.
However, all this changes if you take the American driving test and get a bonafide American license. You can then buy an American registered bike and get insurance within seconds - very reasonable T.P.O.rates!
Again, laws change from State to State regarding the test, but having an official address for all paperwork ( your friend's ) opens many doors. He/She just need to phone the local police/testing station to confirm what you need to do. ( I only had to sit the theory, for example )
It'll probably be far more cheaper and less hassle ( shipping papers, fees etc ) to go over and buy the bike there. There ARE insurers willing to insure you on a British license, American bike - I think I was quoted 150$ as opposed to 45$ with American papers. Hope this helps some!
P.S. Whatever happens - go to the U.S. and tour by bike. Beautiful place.