I wouldn't trust that shit. Don't ever combine ambien and an opiate. It's more dangerous than a benzo and an opiate, even if you are tolerant to both. Also, think about this for a moment. Who the fuck is going to take their time to acquire and extract ambien pills and fentanyl? If there's any possibility of fentanyl I would stray away. I think the fact that your buddy informed you of this was a sign that you should stray away from this particular source.
However, I do understand the mind of an opiate addict. If you do decide to purchase and dose any of this, smoke a smaller dot than you would of just pure fentanyl from a patch (as in, smoke a dot about the size of a 1/3 of a match head), and see what happens.
See if your pupils constrict or dilate. If they constrict, there's more fentanyl (or some other opiate) than ambien.
If they dilate, there's mainly zolpidem (or some other drug that induces pupil dilation).
I know from experience that the pupil constriction effects of opiates usually beat out any dilation effects of any other drug.
Also, to answer your question, no I've never heard of opiates cut with ambien. Why would a dealer want to kill off his customers and hence his revenue stream?
In conclusion-I simply would just not fuck with this source, no matter how much I was jonesing.