I don't understand why you would want to be "friends" with this girl. It's only going to keep you enmeshed with the guy.
If you felt the need to warn her, you did that. My suggestion is that you leave both of them alone and work on moving on. Their lives are not your concern.
I know it's not easy to simply "move on" when you have been deeply involved with someone for a long time, and it doesn't just happen right away. But you can help the process along by resisting the urge to call him, to figure out what he's "up to" or who he is seeing and what they are doing. All of that just keeps you stuck.