In a lot of outsource agreements redundancy liability is often with the receiver of the services.
I assume you are factoring this cost into the overall charge for the services, so I guess it would be a moot point.
It might be worth adding a clause to your contract (if it's not there already)...
The employee is entitled for certain rights. For example if you were to lose the contract and another firm won it, then your employee would be eligible to transfer to the new firm.
However I don't think there is any particular legal responsibility for your client to take over employment...