I am not sure if he can return it for an AG model and i doubt if apple will install an AG screen from the glossy ones too (after the 14 day period) i have a 2011 glossy before but i managed to return it before my 14 days expire BUT the apple return team once told me, in a specific situation they can extend it for me if i wanted too. so it is best to give them a call like right now before it becomes too late for anything. Plus a full install from glossy to AG will be very costly and not worth it. it is like treating your friends glossy screen is broken such as glass break and need a replacement. to swap for ag screen option with $50 is not going to happen if i am not mistaken.
If you get a good deal from him, you can actually take the glossy ones instead. It is not hard to get rid of glare by moving your viewing angle or where you sit. Besides, the gloss ones have more vibrant colors. (sharpness between both model are the same and extremely hard to notice)
Bonus: i ordered both glossy and ag 2011 before i returned one of them. to my experiment, in extreme lighting conditions from outside, BOTH screen are horrible and not viewable easily. the mild diffuse on ag from extreme light resulted in text not readable at all from normal viewing distance. theres no wrong choosing which screen as both of them look great and its a matter of personal preference.
But if cash is not a problem, go get a new AG model instead.