What is that gonna prove in this case? She said it isn't overheating, and the fluids in the car look good. Doesn't even sound like a headgasket unless it's leaking out the edge which again a leakdown test will show nothing. I agree that a leakdown is a good thing when there is an overheating, bad running problem, poor compression or other suspect issues, but not a leak.
My first impression is that it's leaking coolant across oily residue, where is the leak coming from? front, back, side? Headgaskets aren't really common on LS1s anyway. Is it low on fluid? How much of this leaking fluid did you find?