In the male body, testosterone meets up with the aromatase enzyme and gets converted to estrogen. All men have a small amount of estrogen. If you raise your serum testosterone level with exogenous testosterone, your estrogen level will also increase. Other aromatizeable gear like deca or d-bol will convert and elevate estrogen as well.
Gyno is caused (usually) by excess estrogen in your system. That's why (most) women have tits and (most) men don't. Everyone is different as to how much estrogen they can tolerate before they grow tits.
Since your friend has gotten to the point where he is (most likely) above his tolerable level of estrogen, he has two choices:
1.) Use nolvadex to block the estrogen from activating receptors in his breast tissue. This is what nolvadex is good at; it's normally used to block the estrogen receptors in women with breast cancer. This choice will not lower estrogen levels at all but will alleviate gyno.
2.) Use arimidex or femara to prevent the formation of the aromatase enzyme which will prevent estrogen from being formed. This option will reduce the estrogen in your friends system but it takes longer to have an effect on gyno because it takes a while for the existing aromatase and estrogen to clear from your bloodstream.
That is the reasoning behind using an anti-E for gyno. If your friend doesn't want to do anything about it, then start shopping for a nice 44B bra to give him on his next birthday.