The auction started: 2♣ - (2♦) - 2♦
I was called to deal with the insufficient bid, so I took offender out of the room and asked what was going on.
He said his partner's opener was an Acol artificial near game force, and he hadn't seen the intervention. I asked about their agreed responses to 2♣, and was told that 2♦ was a relay. I asked further whether all hands respond 2♦ (if next hand passes), and was told, yes.
If the insufficient bid is not accepted, do you allow any substitution without silencing South?