It's probably just juices from the stems being drawn out and settling at the bottom of the pot, where the extreme heat burns them. I don't want to say your buddy is full of it, since I've never tried it myself, but I find it pretty unlikely that he's getting much "hash" from stems... unless he's boiling a bunch of them at a time, in which case, it's probably just residual leftovers from the buds that once adorned them. If he's desperate enough to boil stems, it probably wouldn't take much to get him toasted, anyway... and even then, his high might just be similar to a placebo effect.