If you don't think the parents will take kindly to you saying anything, can you at least speak to your friend or her brother, I would not take a harsh approach, but teach him how to handle the kitty if you can, I admire the fact that you are at least trying to do something to stop this and taking some responsibility even though the kitty is not yours', your own mother should be prepared to step in and say something animal abuse should not be tolerated, end of story, if everyone just sits and does nothing it will continue and could get worse, I was thinking maybe the cat was peeing because of the continous stress it is under from this boy, however am glad to hear it has had vet treatment.

If you can take it upon yourself to make a difference in this kitties life, you will be duly rewarded, just the feeling that you helped a helpless creature is the most wonderful feeling in the world, GO FOR IT, and try your best.