Solar installers are not known for winning any BBB awards.  You quote this one, "The reason we sent the 1099s . . ."  So whose name is on the 1099?  The electric company, or the contractor? And did the electric company send a 1099-NEC to the contractor, which would have been the right form, and the contractor's bookkeeper said, "You're gonna have to pay tax on this unless you pass it on to a dumb customer"?  

Was there ever a $15K payment?  Did it ever go to the client?  Or did the client get a break on the price because he assigned the credit to the installer?