I would contact that credit union, and ask for the manager. Then explain that you credit report has been pulled twice from there, without your authorization. Don't go into any more detail then that. Don't tell them all the soap opera stuff. Just tell them you want to know who pulled it and why. In other words make it sound like you pulled your report just to make sure all was well (many people do these days with all the identity theft - read on)
They are probably never going to tell you who, but ask anyway. It will get them a bit more riled up. Anyway, from that point, if they do find who did it, that person will probably be in serious jeopardy of their job for doing so. That should do some great damage between the friend and the ex wife.
The other thing you need to do is watch your credit. You are entitled to a free copy of your credit report once per year. But here's a loophole to your advantage. If you are ever denied credit, you are entitled to a free copy of your report then as well. I'm not going to tell you to intentionally apply for something you know you'll never get just to be able to get a report more often than once per year :dgrin but I couldn't stop you from doing so.
Here's the bottom line. Even if it wasn't the ex-wife behind this, you need to ask yourself why it got pulled at all, and that's why you need to check your credit. Right now I would wait a couple months. If anyone has used this info to start obtaining credit in your name, it may not show up yet, as these things get reported once per month, not right as soon as it happens. So you need to give it time for things to show up.
If anyone does start opening credit in your name, you have a really damn good case against the credit union, and maybe even the employee who pulled the report. She may face criminal charges at that point.
One last note, I wouldn't go accusing the ex-wife either. Yes, you have good reason to believe this, but without a smoking gun, you are only going to open a can of worms. It sounds like this woman is nuts, and trying to make life hell for her ex, and you happen to be caught up in it.