The answer is easy, but to get to it, a little bit of a thought experiment is probably helpful. I say, look to how we define our own left and right sides for guidance. When facing forward, our left hand is on the left side of our body, and the right hand is on the right side of the body. Perspective doesn't matter, and there is no ambiguity.

Now we need to extend this to the bed. A bed has a head, just like a person does. So where would its face be? It seems clear to me, unless you are sleeping on a dead mattress, that the face is clearly going to be looking upwards at the ceiling at the head of the bed. So the left side of the bed, if you are standing at the foot of the bed looking at it, would be on your right. Just like the left side of your friend, when you are standing in front of them and looking at them, is on your right.

Now if you just imagine the mattress to be perfectly spherical and in a frictionless environment.......

(Obviously just having fun with this answer, but it's also the right answer)