Old Friends’ ex

I had a friend throughout high school and we were pretty close but we haven’t talked in two years and her ex boyfriend and I hung out and grab coffee and cut up since we have been friends longer than her and I have I really didn’t see a problem with it. She texted me out of the blue telling me that I should stay away from him and pretty much not to get involved because she would appreciate it. She is still in high school and him and I are both in college I don’t know what to do I feel like she should be mature enough to understand that him and I had a thing and we dated before they did and since I dealt with them dating I feel like she should too. I’m not saying I know it shouldn’t upset her but considering we haven’t talked in years I really didn’t see a problem with it. What should I do?