No.3, when put on foil & melted, will turn dark brown. Did he try to smoke it? I know H3 is rare in the states, but it sounds like thats what he may have. Heroin does tastes bitter anyway & No.3 does not dissolve in water on its own & you need to add citric acid to it to break it down. This could be why it leaves something behind when cooked up. H3 can also be grey in color.
I would recommend that your friend trys to smoke it off the foil & see if it had any effect, though it can be tricky to do. To much heat will cause it to bubble & burn, you want to get it hot enough so it just starts to run down the foil, almost as if you are pushing it from underneath with the flame.