Help with likely child abuse

I will try to keep it simple here. Someone messaged me online, asking if i wanted to buy pictures of his younger sister, implied explicit. I continued the conversation until i got his discord, then ended the conversation. I stalked his accounts and i know this person lives in maine and his likely highschool, and theres some more info available.

I need help in, i dont really know what to do. I dont live in the USA, i was only able to screenshot the second half of our interaction, where only "pictures" and "risk to myself" (the user) are mentioned. I feel this is too weak to contact authorities, but i need to atleast contact someone close to him, keep that girl safe. Thank you for any help here.