Discussions

My daughter has spoken to my wife and I about a problem one of her friends at school is having at home. Both children are in grade 6 and her friend is new in town and at the school. My daughter says that her friend was crying in gym class the other day. When she asked him what was wrong, he told her that his dad had been hitting him in the stomach to "toughen him up".  
 
Now, I don't know this boy, but I know that my daughter is a very empathetic child and this is really bothering her. She promised to not tell anyone what she had been told. From what I understand, the boy's parents are separated or divorced. I don't know what the custody arrangements are. He has also been telling the same story to other children in the class.  
 
Here's the problem: I know that something must be going on. Either this boy is a chronic liar or he is being abused. Who do I talk to? My daughter gets very upset at the notion of telling anyone - she doesn't want him to get in trouble and she doesn't want to break a promise to her friend. Is it appropriate to go to the principal of the school? I've never had to deal with something like this before and I'm at a loss about what to do.  
 
Can someone who has been in a similar situation offer any advice?