Can Fortekor cause blindness in cats?

My sister went to to the vets about a fortnight ago for the family cat's heart murmor, he was prescribed Fortekor and she started giving it to him after which he started acting funnily, eating his (clean) litter, being more distant than usual, and wanting to go outside more...especially if it was raining. This culminated in him becoming quite disorientated, we then worked out is that at best he could make out basic shapes visually now, poor guy was even scratching at the side of a door to be let out...it was already open. Obviously the medication was stopped. She was concerned when looking at the bottle that it was 2.5Mg and that was what our dog was on when he had a heart murmor, & he was easily twice weight of the cat-so was this too much for the cat? His eyes are clear if a little blank, he doesn't respond to moving stimuli ie. moving finger etc. His eyeight was totally fine before the medication and now almost totally blind understandably this is quite distressing. Thoughts?