Saw my cousin having sex

She's only 12. Her 13th birthday is in two days and this was at her party, I don't believe she saw me, but she's still only a baby... It might've been an older dude but I'm not sure because a lot of guys at her party were 13 and very tall. I don't plan an telling her parents but I do plan on talking to her, giving her protection, and telling her she should wait.... is there anything else I should do? (If its an older guy I'd probably tell the parents)