Because you have reserved a twin room/single use and not the small single with one bed, there are two beds in the room. Your friend can stay in the same room for the price difference between a single and a twin - $25 per night.
We only extra people in the room if a bed is available.
“Rooms are quiet, you do have some planes flying over every now and then,
but you'll be too burned out from visiting to even hear anything while you sleep in cloudy softness! Sometimes, the air conditioning from a neighboring room vibrates the window...but a little piece of cardboard fixed that pretty fast!”