If they know someone who is being sexually abused then they need to understand that they may have a legal responsibility to report the abuse. In many states, mandated reporting laws make it a crime for an adult to refuse to report suspicion of child abuse of any nature. If they suspect that a minor is being sexually abused (or otherwise abused), then they need to make a report to the appropriate agency.
Even if they are not legally obligated to report the abuse, the safety and well-being of the child should be the overwhelming guiding factor in their decision to report. They can find abuse reporting numbers for their state by visiting this site. Most states allow for abuse reports to remain anonymous.