if hes an honest mechanic he would have suggest half cut in the first place, cut the cost to half, looks like he want to make a quick buck.

Fyi timing chain engine rarely fail due to chain failure. If timing belt, that is more believable. Chain is lubed constantly. The only thing that bugs timing chain is the tensioner. Which is oil pressure activated. Even then the chain wil not break but would slack and be noisy.

But you did mention non service for long periods. Which would mean parts could overheat and jammed. But no one could know what went wrong but only a guessing game really.