Leak from a condemned drain

[Wonderin King]

Hi everyone, 

We noticed a drop in our pool's water level in October last year, but the reduction was not significant. In March, the safety cover was almost down at the bottom of the pool. We called our pool company, who installed our liner two seasons ago, to check the issue's source (assuming that is a seam separation). The company sent a diver yesterday to check on the issue. The diver checked everything and finally concluded that the leak was from the (condemned) drain. The liner under the cap was ripped.
Further investigations showed there was no plug installed on the line under the cap. The diver also said something about no sand being under the cap either. They did install a plug and replaced the cap, charged us $600 and left. Right before they left, we asked if the liner installers were supposed to place a plug on the liner at the time of installation, knowing that the drain was condemned. They hesitated to answer and finally said that they don't know. This issue has cost us a couple of thousand dollars as the safety cover is completely ruined and we have to order a full load of water which would cost us at least $400. Now does anyone know the answer to this question of ours: The liner installers knew the ldrain was condemned (we ask them if that could be fixed, and they said it would cost a lot, so we direct them to treat that as condemned); should they had placed a plug on the line on the drain side?

[RDspaguy]

@jimmythegreekand @Pool Clown are the pool gurus here. Maybe one of them can say.

I would think they should have, but I have never done a liner install or a drain elimination that didn't involve concrete.

[jimmythegreek]

The drain should be filled with concrete amd flushed smooth amd the liner placed over with no penetrations. 

At the minimum they should have just left liner intact and installed the cover grill. No reason to cut liner I'm on an abandoned drain