I know the feeling. The first time I fished the Chattahoochee it was mid-August and about 95 degrees. We drove over from Birmingham "to catch us sum trout".
We pulled in just below the dam with our spinnin rods, 12# test, and shorts, an proceded to wade out in the river. After about 3 minutes I couldn't feel my legs anymore and they looked like I had just worn a new pair of Levi's in hot water. Needless to say it put a slight damper on the fishing. We were able to catch us a cooler full of fish, but I definitely learned a valuable lesson on tailwater temps. I have grown very fond of my waders since then, and always keep them handy.
Anyway, I don't know what kind of fly that is your describing, but I'm sure the man who gave it to you was a good fly fisherman, he wouldn't steer you wrong. Hopefully you'll get a chance to get back to the Clinch and check it out, and give us a full report. Pack the waders, too