What will you do if a client wants to have you as his girlfriend?

Politely decline him and refer him to the manager

you can simply answer that my job is not permit me to do that also you can speak to my manager to help you

I would politely decline while boost his ego, for he is a client and he should be happy. Also I will assure him that the woman he ends up with will be the luckiest. Kind of like this "I would love to, Mr.Rajesh but I am not ready for a relationship. Also my job does not allow me to have personal relationships with the clients. I am so sure the woman who ends up with you will be the luckiest woman in the world. Unfortunately I am the unlucky one. "