Would you have done this?

My last relationship was not good. My ex frequently gaslighted me, was emotionally abusive, and did a number on my self-confidence and self-esteem. We broke up a while ago, but in August he asked me out again. Of course I said no, which angered him and he proceeded to gaslight and insult me. He even reached out to one of my friends and started flirting with her, I guess as a way to get back at me. Obviously my friend shut that down. He left me alone after that…until two weeks ago when he reached out again, but I immediately ended the conversation and haven’t heard from him since. Since we’ve broken up, I’ve struggled to trust other people because of how emotionally crushing and draining that relationship was. 
Yesterday, I found out he has a new girlfriend (K). This girl also happens to be someone I went to college with. And although we weren’t close in college, I still wanted to say something. Not in a vindictive and crazy ex way, but in a “hey keep a watchful eye out” way. Of course, I didn’t want to reach out directly because then I would definitely come across as the crazy jealous ex. So instead, I reached out to one of K’s best friends, (let’s call her C) who I am closer with. I didn’t give C specific details. I only said that the relationship took a toll on me and that C should just keep an eye on her friend just in case. C immediately started thanking me, saying she was wary of him and didn’t get a good vibe from him. C also told me that K and my ex have only been officially dating for a week, and talking for a few weeks prior, which they both said was exclusive since they hadn’t really been talking to anyone else before that. (Which if they were “exclusively talking”, why was he reaching out to me?)  I reiterated to C that I wasn’t trying to be nosy or sabotage the relationship, especially since their relationship might not turn out to be anything like mine was. I just wanted someone in K’s life to be there in case things turned for the worse. C thanked me profusely and said that I was a girl’s girl and she understood that I’m simply looking out for a fellow gal. C told me she would definitely keep an eye on it, especially since she was already skeptical about their relationship. C also asked if I would prefer my name not be mentioned if the conversation ever came up with K. I asked her not to mention me, in fear that if my ex ever found out, he would take his anger at me out on K. C was extremely grateful and understanding about that and said she would let me know if anything ever happened. 
Another friend of mine saw the photos of K and my ex on both of their Instagram stories, and reached out to see if I was okay. I told my friend I was okay, just worried about K and told her about the conversation with C. My friend told me that it wasn’t my place to have done that. She said “yes he’s an asshole. Yes what he did to you was incredibly wrong and no one should have to go through that. But you don’t know her that well. She’ll figure it out on her own, hopefully sooner rather than later. But it’s not your place to meddle”. I told her that that’s why I reached out to C instead of K directly. Because I didn’t want to cause drama or stick my nose in their relationship, I just wanted someone to be there for K if something happens. And my friend told me she still wouldn’t have reached out. 
I was just trying to look out for my fellow lady, but should I have left it alone? What would you have done?