OK, only pay attention that phone is in good condition. And prepare Tempered Glass if owners didn't use, but I think now mostly people use Glass protection.
That's most important, if he worked few months without problem 99% will work long time.
Space Grey and Silvers are obviously nicest colors because we are males, but it's obvious that someone really smart chose colors for phones. If Apple launch White iPhone 7 I think that would be enough and they should stay with his colors. Samsung colors are terrible... special models with little holes on back side, terrible.
6s is great for first iPhone, people complain about antenna lines but from my perspective is not bad at all.
This is your new phone... it's beautifull.