I've said a couple of times before that Phil is definitely, 100% gaslighting her in a textbook fashion. I have no doubt the relationship is purely transactional for him. He pays for her shit and gets laid every now and then, she cooks, cleans and doesn't get ideas above her station. That doesn't mean Kat isn't aware of that however, and isn't gaming Phil herself. God knows he is an easy mark if you know what you're dealing with. So any advice you give to her is based on the assumption that she doesn't know what she's getting into, and that she does indeed have pure intentions towards Phil.
With that proviso, my main piece of advice would be to do your research. Fact-check everything he says. If he tells you the date, go out and buy a newspaper to make sure. Phil will lie about absolutely anything to get his way. If he insists you can't get a job, or go out with friends when you make them, or says you can't do this and that because you need a new drivers licence, then you check that shit with reputable sources. Keep in touch with any friends you may still have at home and double check everything. One of the key parts of emotional abuse and gaslighting is making the victim doubt themselves, and to convince them that abusive behaviour is normal and they're wrong to be upset by it. Make friends, read up on shit, and always remind yourself your wants and your feelings are valid even as Phil protests they aren't. External support structures will get you through, you just have to trust in them. Easier said than done when this type of abuse means you don't even trust yourself, but still.