Can my friend get in trouble for starving horses that aren't his?

If he does not call the humane society, they will end up blaming him no matter what he says. I would suggest you call and have your friend give them all the information on this girl and have them work it out with her. For now, you should feed them whatever you can spare, if you can. If you have a local feed and seed or tractor supply they should have some food. And take pictures of them for evidence in court.