You say new Xs. Xs was discontinued last September. The warranty does not begin on the day of activation. The warranty begins the day the phone left Apple Inventory, which could have been months ago.
Your friend will want to make sure the phone is not Activation Locked. It should open to the hello page. If it doesn't, they'll be buying a useless brick.
In terms of determining if the phone is refurbished, he can go to Settings > General > About and the first letter of the model will tell the following:
M= new device
F= refurbished
N= replacement
P= personalized
Send your friend the following user tip I wrote about used devices --> The All Too Common SAD Reality of Buying … - Apple Community
Finally, your friend should be VERY cautious as it's very common for people to lose their money and get useless devices. The best advice is to buy from authorized sellers.