In all the years of riding dressage I've never heard the term "reverse shoulder-in". In my mind, I would equate that to renvers but by how you've described it that doesn't sound like what you were doing! Shoulder-in, "reverse" or otherwise, is a completely different movement than travers.
In basic terms, shoulder-in you are bringing the horse's forehand off the track while keeping the bend around the inside leg. Travers is bringing the haunches in, shoulders travelling straight on the rail, while maintaining the bend around the inside leg. Two different parts of the horse are being manipulated, so I don't think that the reverse shoulder-in you were doing and travers are the same thing. It could be thought of as the reverse of shoulder-in as you are manipulating the haunches rather than the forehand, but I wouldn't equate it to reverse shoulder-in.
When I'm riding renvers, I start in shoulder-in and then gradually change the bend from around the inside leg to around the outside leg. I keep the position of the horse's shoulders while I change the bend to the other direction. Since the shoulders are in same postion in shoulder-in and renvers, but the bend is opposite, this is why I would think renvers = reverse shoulder in.
Does that make sense at all?
Unless she was simply asking you do do a shoulder-in in the opposite direction of travel - shoulder-in right while travelling on the left rein for example? In this case I could see how this would help with a horse overbending to the inside/falling over the outside aids; it would change the horse's bend to the outside and correct falling over the outside aids by making him more upright. I often use counter-bending/counter-flexion to help correct a horse falling over my outside aids.
Sorry I can't really answer your questions as I'm not entirely sure what exercise it was that you were riding! I'm kind of thinking out loud here and am interested in understanding what exercise you were riding