If I were to use a business card, I would just fold the card in half, and then put the card in the gripper correct? Also, is there any chance of string slipping while you have a business card in the gripper?
P.S I thought about just not stringing his racquet but just told him to see how it feels. If it's bad and he does not like it I told him I'll string it again for free. Besides, he did not even pay 7.00$ for this string job, so I think he was okay with it.