If you want to flop (the correct term) an image, you can scale it -100,100 (note the minus) by changing the scale property or by using the distort>transform effect. If you only want part of the image, you mask it. The technique is simple, but it sounds as if you want something different.
When you look at a person and a reflection of that person obliquely, say from close to the wall holding the mirror, the person and reflection will look similar. However, If a person stands on a mirror and you view them from above, you see the top of their head, but in the mirror you see the bottom of their chin. Is that the problem? That the mirror should show an image that is actually different from the footage, not just a flop? If so, that cannot be done without shooting more footage.
Keep trying to figure out how to tell us what you want. Every VFX person needs to be able to express clearly what he/she wants in order to determine the proper technique to use.
Steve