Even though he's younger than 18 and cant be charged with , can he be charged with something?

Yes, he can be charged, and you should tell an adult that you trust like your parents, a school teacher or principal, even the school nurse. Your friend is being molested, and it is against the law, and she needs help. It is great to keep a secret, but not when someone is getting hurt. Tell someone right away, your friend is probably to scared to tell anyone, but you should.