I could be wrong but it sounds like motor oil. If it is, He shouldn't drive it again untill he adds new oil or he'll damage the engine. Also, he should check the manual and find out the proper oil to place in. You don't want to add 4-stroke oil in a 2-stroke engine or vice versa. If it's 4 stroke pay close attention to the ratings (i.e 10w-40, 5w-30, etc.) You don't want to add in the wrong type.
By the sounds of it, he needed to change the oil anyhow. Remember, bottom plug to drain, top plug to fill. Remember to replace the bottom plug. Was the plug metal under the engine? If it was under the engine (normally on the right side of the bike), then it was definitely the oil.