well the truth of the matter is that if dad has a problem with alcohol he is the only one who can fix it.
i know you are concerned and disppointed in what's been going on but also you have no power to change him.
the only other thing i could suggest is if the rest of the family agrees you all could do an intervention and usually the is best done with an addiction therapist. otherwise it/he could cause havoc and mayhem and i know that is not what u want to do.
as for his lady friend, he has the choice on this too tho u may be concerned or not like it.
it might help you to accept things as they are right now and get on with your own life. worrying, and i do understand that, will accomplish nothing constructive re this. his behavior and poor chlices prob stem from his alcohol use tho i am not sure. alcohol will skew our perceptions and help us make very poor decisions if we have drinking problems. sounds like he does.
so an intervention may help but no 100% assurance if he chooses to continue drinking. that may be what he does. usually one has to hit a bottom, be it emotional or jail or finances, etc.
hope this helps.
oh and welcome to pc! i'm glad u found us.