really hope you work this out as it sounds like an awful situation for you.
would it help to print this thread and show it to him?
i have a 22m old monkey who can drive me nuts on occasion. at this age, as you know, kids have no real concept of danger and are just learning about boundaries etc.
my dd needs watching 24x7 otherwise she'd be swinging from the chandeliers with delight and no thought whatsover for her safety!
i also find that my mood can directly affect dd's behaviour. when i am grumpy and stressed she becomes more demanding, less patient and more likely to act up.
when i am calm and take the time to explain firmly why she can't do something at the same time as offering a distraction she is a different child.
whether you are pro-smacking or not, at this age it really serves no purpose IMO (I'm not pro-smacking myself btw). They're just not old enough to understand punishment of this nature.
but you know all this so i will stop waffling on.
maybe you could just try to encourage him to look at the benefits of staying calm but firm without resorting to the smack.
ask him if he really enjoys physically hurting his daughter. i can't believe he would say yes and assuming he doesn't say yes then show him the other options.
good luck and well done for standing up for your daughter and your beliefs. i understand your comment about "my way or the highway" but ultimately, you know you're protecting your daughter and breaking the cycle.