15 year old dating a 24 year old??

Ja
My cousin, she just turned 15 in October is now dating this creepy 24 year old man. 
Before anyone tells me "it's not your business." I feel that if I fear my cousin may honestly be in danger or  "in love" with a predator, that it is indeed my business.
What makes me think he just might be a predator? Well first off, my cousin was planning on saving herself for marriage, and within a week of dating, they had sex. The police were notified, but since her parents consented (complete bs) nothing happened! 
Also, let's mention they met on Tinder, I'm the only one that knows bc she told our family they met at the movie theater. 
She's showed me their texts and literally every conversation goes like this 
Her- Hey!
Him- Hey sexy! (Gross)
Her- What are you up to?
Him- Nothing, just horny. Can I get a picture of my baby girl? (Nudes)
It's revolting, yes I know it's child pornography, yes I've told the police, but since is over snapchat they have no way of proving he ever had any pictures. So in other words I just looks like I'm a crazy jealous bitch. 
Also, whenever they're with each other, he just stares at her very creepily. It's weird and makes me feel uncomfortable. I've tried to talk to her, and all she says is "You just don't get him like I do."
This situation is just all kinds of fucked up, and idk what to do.