I feel that when God is telling us to do "something" or to "help someone" he will engineer events such that we are at the right place and right time to help.
It is one thing for you and her to constantly be crossing paths and events align in such a way as to provide the PERFECT opportunity to talk to her about God.
It's an entirely different thing for you to not even be in the same circles anymore and you feeling the need to engineer events yourself so you can talk to her-- i.e. like calling her up out of the blue.
It is my belief that God works through us, but there are a lot of us. So he will choose the right person as a vehicle to reach her, and that person will be put in the "perfect place" at the "perfect time" for her to hear his message.
If that person is meant to be you, then you will constantly find your life intersecting with hers in situations perfectly positioned for you to talk to her about God.
Here is an example.
I went to a casino with some friends and we were playing the various games. I got super lucky and won $550.
The very next day, I'm at the grocery store and there is this elderly woman a couple of spots ahead of me in line. There is this "helpful" young man next to her which I sorta assumed was her grandson. She takes a couple of steps over from her cart to peruse the Magazines at the front of the counter and this guy grabs her purse and bolts out the door. Just like that. She starts screaming, "My purse, my purse"... It was so weird, it felt like a cliche comic book moment where the criminal grabs a purse and runs and some do-gooder hero type stops him...
But in this case, that didn't happen. He had a clear line to the sliding doors and given the layout of the checkout counter, lines, etc, myself and others were boxed in and unable to give chase. And of course the dude was lightning fast.
The police are called... In the meantime I remembered that I forgot a few things so I left the line and went back to shopping. I come back 7 minutes later and the police are there along with the store manager. The lady is crying and saying she is on a fixed income and had her rent in her purse and that she is already a month late on rent and she will get evicted if she misses rent again. SOmeone asks her what her rent was and....
she says $550.
When she said that, I felt like a bolt of lightning hit me. The hairs on my everything stood up, tingles ran up and down my body. It just so happened that I was in the exact right place at the right time in the right circumstances. Had I gone to the store one minute earlier or later I would have missed this. Had I not have forgotten those items I would have been in another part of the store and completely missed it. I knew, without a shadow of a doubt what I had to do. I walk up to the lady and ask, "How much was your rent?"
She answers "$550".
I reach into my wallet and that $550 is completely separate from my other money. I hand it to her. She says, "Are you sure???, I don't know when I'll be able to pay you back."
I reply, "No, don't worry about it, that is money I won at the casino yesterday, odds are if I go back to the casino I'd just lose it anyway, so you might as well have it. Seriously, don't worry about it."
IN any event, that is how I see these things. It is my belief that when god calls on you to act, it is very deliberate and there is no doubt. But beyond that, things will engineer themselves to put you in the right place at the right time to help. Now if that is happening with you and your ex that is one thing. But if it is not, then it is just wishful thinking on your part (again, just my humble opinion on the matter)