The cough happens immediately after injecting. It happens when the substance enters the bloodstream too fast that is why it is more common with acetate than enanthate. It will also happen when you nick a vein obviously letting way too much in too quickly. There are also areas that are very rich in minute blood vessels those areas will suck up the substance too quickly and cause the cough. The body will send more blood and blood vessels to areas that are traumatized. That i why I say it is best to rotate injection sites try not to hit the same place more than once every two weeks. I have seen may many times someone developing tren cough during the course of a cycle injecting the same spot over and over that spot gets more rich in blood vessels as the body tries to heal it and thus causes faster absorption.
Cough does not only happen with Tren but is much more likely... and I'm amazed he's still injecting if he's getting it every time that cough will put the fear of god in you and make you balk off a pin or even a whole cycle.