My client was refused trv recently, but according to him he called his who is a permanent resident and the sister involved a member parliament in the case. My concern is can the refusal be overturned?
An MP cannot influence or overturn a decision.
You / your client can submit a case specific request to ask for reconsideration. However IRCC rarely considers these requests.
Your client can appeal the decision. This will require your client to spend $3-5K to hire a lawyer in Canada to represent him - with no guarantee of success.
Or your client can reapply.
You should really know all of these answers if you are providing advice on immigration matters. You shouldn't have to ask these questions on a forum.