I'm going to bet you didn't mean so say you were going to remove the subfloor. I think you were told to remove the underlayment.
I don't understand about the floating part though. If he's planning a mud job, you don't need the 1/4" backer first.
If the 'shower' is a stall as apposed to a tub/shower, the details can get lengthy. If there is a membrane behind the wall board, (plastic or faced insulation), he should not put a membrane over it too. It would be better to do the mud job, then apply a membrane over that.
Please reply if it is a stall shower as there is more to that then a tub surround.