It sounds like the pipe had served its useful purpose and it was time to replace it. If anything it was a blessing to fail now before you were done with your renovation. If it was my home I think I would have replaced all of the 80-year old pipe anyways. The piece the plumber replaced was one section of many which is likely ready to be replace.
BTW: While I am not a Lawyer I have worked as a consultant (Expert Witness) for construction litigation related to plumbing and mechanical issues for the last 18+ years. In my opinion it does not sound like anything the Plumber or the GC did contributed to the failure of the pipe.
"Somewhere a Village is Missing Twelve Idiots!" - Casey Anthony
I never lost a cent on the jobs I didn't get!