He tried to start it and all it does it make a vibrating noise from the stator (relay?). I replaced (happened to have on hand!) the stator solenoid and the starter but it is doing the same thing. I read that it could be the battery but I tried to jump it off to one of the other bikes and it didn't help. Any ideas?
It sounds to me like voltage testing is in order. I'll guess that since you dragged out the jumper cables before the volt meter that you are saving the volt meter for a last resort?
Start by verifying a good complete and sound circuit for the starter. Verify the following. Battery positive post snug and clean connections, the starter relay connections are tight and clean, the positive wire on the starter is tight and clean, the starter and engine are clean where they bolt together, the engine ground wire is tight and clean, and the battery negative cable is tight and clean.
If all of the above are verified and it still acts the same with a different starter and a different relay, with or without a jump start, then you're gonna need the volt meter... The buzzing BTW indicates to me that you have at least partial continuity throughout the circuit, that means that a test light is going to give you results that are false. Or technically "true but misleading".