Should Should reach out to my ex or tell his baby mama?

I was with this guy for 5 years and he left me to try to make things work with his ex again. He was cheating on me with her. Surprise didn't work out but I'm done with jom. However I had to pick up my baby sister from this guy's house because she was drunk and scared. Many people were there. I see my ex's son(He's 15 same as my sister). Now he was a shitty boyfriend but loved his kids. His baby mama loves them too and his son was never a bad kid. Just depressed because of all his parents drama. I see him cuddled up to some guy that looked between. 18-20. I'm trying to get my sister and he doesn't see me and this guy is saying "Just try it. It will make you feel better. I promise it's not a big deal". I didn't hear what my ex's son said but I saw the man tie something around his arm. My sister starts vomiting and I'm trying to help her, then I see a needle INCHES away from his arm. I go hell no and walk in grab him. He seemed shocked to see me. He knew my sister was here but didn't know I was. I drag him and put him in my car. I'm going off on hoe stupid that is and he was saying he's just tired if everything with his parents and wants the pain to to away. I drop him off and he said "Are you gonna tell me parents?" I said idk and he begs me not to. I get we aren't together but this seems like something his parents should know.