Talk to them.
Tell them what you think.
If they don't listen, then...uh, try and help the animal if its not harmed too badly, and uh, take it to an animal hospital.
If helping the animal is not an option, tell a person who is an expert on animal health stuff who you know and who your friend knows to talk to them about it. If they still don't listen: Tell them not to do it again or you'll tell the police.