Phone socket wiring

[#1]

I am trying to advise someone who wants “Standard” Broadband, not fibre. The existing phone wiring comprises an old style BT Master Socket, JL 2/1A, with an extension wired from that to another socket. The lady wants to run from the extension socket.
My concern is that if there are problems with the internal wiring she will not be able to isolate the extension and test from the master socket, in the way one would with an NTE5 socket.
I am assuming that the original sockets were installed by BT, does Openreach have any responsibility to replace old installations with NTE5?
Of course it may be that the present wiring is just fine and will work with the appropriate splitters plugged in to the sockets. However if this is not the case I am worried that Openreach’s response would be to charge a fortune to rectify the situation.

[#2]

The broadband is only guaranteed from the master socket I’m afraid. I see no issue why and extension wouldn’t work, but cannot confirm it, especially as I haven’t seen the quality of the writing.
Should a fault occur, an engineer would be required and he’s update the socket as standard.