your friend is experienced in being in idiot, perhaps.
mdma = increases your heart rate
oxy = lowers your heart rate
both also affect the respiratory somewhat because of its side effects on the heart, so in short, DO NOT MIX THE TWO. either your friend is lying that he did those two drugs together, or he is a very lucky man to not have had anything more serious happen under the influence of them together. i once thought i was slick to take oxy on the comedown, then found myself vomiting all over the bathroom about an hour later.