I believe the question he is asking is can he join the client company that he was working for through the consulting company. He is not talking about at will contract. He is talking about non-competition contract in which you can not join a client company that you know of due to your relationship with your original company.

This is the thing you might want to investigate or take a lawyer's opinion. This could be a thing that may go against you depending upon the situation. If your consulting company doing a lot of illegal things, then they might not pursue it. However, if they are running like a US based company, then they can certainly come after you.