two ideas, one cheated
1)pay fro materials he gave, finish guitar and play it in work, make him see what did he lost make him jealous...
2)not so good, screw him and make him quit the job, or quit the job and start live like real rock'n'roll dude
ps: i liked idea of beating him with guitar, but guitar have done nothing bad, so pick something else to hit with(jackson stile neck)