Absolutely rude!! Even with dietary requirements its a bit odd, I have a few food allergies and wouldn't send someone a must use recipe, possibly ask they not use cream or something I'm highly intolerant to but if it was that big a deal I would just do it myself rather than hand out direct orders!!
I would make whatever and tell them your shop sold out of every single ingredient from the must use recipe, must be the Christmas rush on certain items in your area, nothing you could have done other than substitute their recipe for one of your own