It probably will hinder your gains as estrogen is responsible for some of your gains(not only in the form of water). Also it lowers igf-1 levels, which may also hinder your gains. IMO arimidex will slightly hinder gains, but not by that much. I prefer to take l-dex throughout the cycle to stay hard. For your friend depending on how much test he is taking, he may not even need to use arimidex, but I would highly reccomend it, if he is prone to gyno or on a pretty high dose of AAS.