Probably no need to replace the clutch arm, just disassemble it, clean it, and give it lots of sticky bearing grease when you reassemble it. When everything's buttoned back up, adjust the clutch. And don't forget to grease both barrel ends of the clutch cable -- a binding barrel end at the hand lever is a known cause of clutch cable failure.
I agree it is a strange failure -- one I've never seen nor heard of before. Perhaps a freak defect in the part, not "caused" by anything in particular?