the important thing, looking long term, if you are close to the patient - do everything you can for them.
I can't emphasise this enough- especially your wife as it is her dad. She is in for a rough ride emotionally.
Make time for them. Be kind to them. Mop up the vomit with love & tenderness. Clean up the poop and all that goes with it.
Because when they have gone, one of the stages of grief is guilt- and if you have done everything you can, you get an easier ride on the guilt trips and can then focus much more easily on the happier stuff.
Also do daft stuff to alleviate the gloom.
I would walk in on my gran as she was sat bored watching TV & flash my boob at her or moon her with my ass- random. Be playful as much as possible.
Her response to these things is what I eat my best giggles off:
"Ya daft BUGGER!!" "Tha gormless" (Broad Yorkshire)
Also I found food was a major thing- I would often cook up (*slave for hours in creating this wondrous meal) what was her fave meal (at her request) & find she would take a taste of it then ask if we had any tinned salmon or egg & chips.
I like to think it wasn't out of pure contrariness (though most likely a 50:50 split) but because the appetite goes & it takes a lot of effort to get nutrients in.
Also don't be hard on yourself, it is natural to feel angry, frustrated, & a whole spectrum of negative emotions- if you do vent at the patient just be sure to say sorry & that the pressure just built up. None of us is superhuman, we all have our limits. Palliative care in the home sure shows us what those limits are in a very short time.