You should look at Penal Code sections 261.5 (b) and (c) which makes having sex with a minor by any person (not just an adult) a misdemeanor if the age difference is less than 3 years or a felony if more than 3 years. If you have some genuine basis for this accusation you could report it to the police or the DA. If you don't you should go back to minding your own business.
If the 17 year old is more than 3 years older, it is a felony. If the 17 year old is less than 3 years older, it is a misdemeanor. It is a serious crime that can cause a lot of issues in the future, such as sex offender registration and mandatory sex counseling.
Seth Weinstein, Esq.
Practicing throughout Southern California
This reply should NOT be considered a legal opinion of your case / inquiry. At this time I do not have sufficient factual/legal documentation to give a complete answer to your question and there may be more to the issues you raised then I have set out in my brief reply.