Guess I should have phrased it better - if we believe Phil about keeping the door closed for her sake then she's always around because she got fired, if we just look at the fact that he barely talks about her anymore besides things that could have happened 3 years ago that points towards her being gone.
We have proof she was working part time from the bankruptcy leaks. That certainly could have changed, and from Phil's narrative it sounds like she might be working a different job now, but it still sounded like a part time thing. In my experience, which I admit is pretty dated, when you're working part time in retail, you don't have a set schedule. They slot you in when needed, typically around the weekends. But I definitely had plenty of times where my schedule varied week to week.
But that being said, I certainly don't think it's impossible she got fired. IMO, I'd throw in the possibility she quit, given how much she was supposedly complaining about not being able to get a promotion. We can pretty confidently say she likes the hermit lifestyle, and with Phil footing all her bills, there's really no reason for her to work at all.
As far as proof of life, there was the, in Phil's mind "hilarious" water spilling incident however many months ago that was pretty clear proof she was still there. That's the only concrete evidence we have, but it's certainly more than anything saying she's gone. As for him not mentioning her, she still comes up every now and then. It's just incredibly boring, because Khet is an incredibly boring individual that somehow manages to have less going on than Phil.
If Khet decides to leave, I imagine she'd want to formally divorce him. It'd be hard for her to grift her way into someone else's home if she was still married, plus she'd want to get whatever she can out of Phil. I'm 100% confident if that happens, someone in the detractor community will find and share actual proof.