First of all: you said in the thread title that it's your dog, but in the post itself you said it's your dad's dog. Which is it? That's an important detail.
Without knowing that first fact... my general advice is to get evidence of it. Whether that's secretly filming it with a device, or getting a witness, or something else you can think of. Maybe recording a conversation between you and your dad where you ask him why he hits the dog.
After that's acquired, just call the cops or the humane society. Tell them you have evidence of your dad abusing the animal, and you want to secure its safety.
Alternatively, after acquiring the evidence (don't tip your dad off before gathering that) you can threaten to report him if he doesn't stop the animal abuse. If it goes well then problem solved. If he continues abusing the dog even after that then make good on your threat and call the cops or whatever organization is best/available in your area.
It's going to take some guts, but do it for your friend. You know you wouldn't want your friend to sit there and watch you suffer if the roles were reversed. You would want them to summon help for you.
Do your best.