Do your friend, and you if you listen at his house, a favor. Buy him the SL2000 replacement tweeter, the RD0194-1 from Polk. That SL2000 with it's nasty 5dB spike between 12kHz and 13kHz is the worst tweeter Polk ever made. He should replace all of them for the best results.
That said, his 1C's are equipped with a polyswitch to protect the tweeters from clipping damage. They go bad from tripping too may times and just from age, so he should check that the tweeter is the actual problem first.