Gall Blader Surgery in Alcoholic diabetic patient

My father is 65. he is alcoholic and he has been operated for gallbalder by Laproscopic surgery.
he was fine till 6 hours after operation. but suddenly he developed pain and after evaluation it was found that the upper drain in GB fosa has not drained anything but pelvic region is showing fluids.
Drain put up and it drained 1.5 Ltrs.
they shifted him to ICU and his conditions kept worsening.
They than opened the stomach and told us that
SINCE GALL BALDDER WAS GANGRENOUS it eroded in wall of colon and produced a leak. which they have found now and sealed.
My questions are:-
1- COLONOSCOPY done before surgery was showing nothing? if they could not detect it then was it a medical procedure failure.
2- Was the puncture created by Doctor During Surgery?
3- Why is a leak cant be detected in time so that my dads conditions worsened.

they keep blaming it on alcohol and gangrenous bladder.
I DONT BUY THIS ARGUMENT

[crambone]

colonoscopy can EASILY cause cause teas in colon!,,,that is a very dangerious procedure,many people take for granted ,like it aint a thing!
id realy try to avoid that myself if a can in my future...
problem is,,PROVE IT WAS THEM!!!..they got good lawyers that will say otherwise,and usualy you have to sign a reliece that accnowledges this COULD happen.