You can't just fit a cat flap to double glazing, the whole panel of glass has to be removed and replaced with a sealed panel with a hole in it suitable for the flap.
Can't they make the hole to fit the cat flap? We put one in a door but replaced the bottom glass panel with a solid white one which was then easily cut for the flap. We used an ordinary Staywell cat flap but you'll need the flap first so they know the size to make the hole.
It's not going in a door, it's going in the smallest panel in an all glass conservatory and they have to replace the entire glass panel with reinforced glass to meet regs. They are putting the hole in the glass when I make it but I need to find a suitable cat flap that will be put in the hole at the same time.