Why do they still have the plan?
Why not terminate it or merge it with their PS/401(k)? Assuming they have another DC plan.
Amendment would have to be prospective.
If the transferring plan is silent on the issue, then I would say that such a transfer is not permissible. Take a look at 1.457-10(b)(5) - one of the requirements is that the transferor plan must provide for the transfer.
Both the sending and receiving plans should have terms for transfers. If the sending plan does not have terms for a transfer, how can you tell that departing funds are not a distribution, which is the only thing the plan describes that fits with departing funds?