Hmm, that is tricky. That's the kind of thing you could fake if you were showing to someone who is not familiar with the room. But if its for the person who sleeps in the room every night, its a bit tougher to fake stuff.
But, if you are putting in a large mirrored wall where they have none, they do not know exactly what the reflection should look like. So just getting close should work.
to get "close" I think you could stand where the wardrobe is planned to go and take a picture looking back towards the base photo. You will probably need a really wide lens - 18mm or so is my guess. I could be wrong though. If you have various lens options, I'd play around with them. And you will probably have to do some image editing on that image as well. It might need to be blended with the backsides of objects you have rendered or something. In other words, this will be an imperfect science.
