My gf has a iP4S and it really is appallingly bad... it lost 30% in standby overnight. Clearly has some big issues with software.
Is it updated to the latest iOS? That has a battery fix that has worked for some people.
It should lose about 2-3% overnight, maybe a bit more depending on how it's set up.
If mine was like that I'd probably get it replaced, as not all of the phones do it... it's an odd one. Apple really need to sort it out as it's affecting a lot of people.