Did you ever stop to think maybe it really is your fault? I mean if you did not keep stringing his rackets he would not break them. Maybe you should suggest he use another stringer to see if his strings and his money last longer. Maybe you could allow his to use your machine with his strings to see if he could do a better job.
I do know this, the longer you let this go on the more you are going to have to support him. Maybe he thinks you just like stringing rackets and he wants to be your friend.
Everyone has the right to be stupid... it's just that some people abuse the privilege