I would give North 90% of the blame. In a casual partnership having agreed to play RKC, I think 4NT sets Hearts as trumps. North had an easy and non ambiguous 3C (followed by 4C and then south can deny interest in C by bidding 4NT, any cue bid would set C as trumps). My only concern would be that 4NT be taken as quantitative (since 2S followed by 4NT is available) but I think unless clearly agreed 4NT is quantitative south should respond to RKC in H. 6C can clearcly be interpreted as bid 7H if you have something in clubs xx or CQ(since N is not interested in finding out about Kings)