Honestly I’d just dm her straight up. Whether on your main account or anonymous one just say hey I know this might come off as strange and we don’t know each other, but I felt it was important to reach out. I was in a past relationship with ____, and over time became physically abusive. I just felt compelled to share this with you incase you’ve experienced any red flags and to be careful. I don’t want to interfere but personally I wish that someone would’ve told me.