Welcome to the forum dark_arts!
Interesting post. Tough one because you seem to love the pug more than your girlfriend or her boyfriend.
I'm really not sure what you can do here...because as you stated it's not your dog.
I agree that they do not seem to be taking great care of the dog but it doesn't sound like this dog fits any criteria of "abuse" to the point where any authorities would get involved. Compared to some abused dogs...unfortunately, this dog doesn't have it that bad. and Unfortunately, this is the plight of many pets. I agree that neglect is abuse btw.
My best advice, if it bothers you that much - might be to ask your friend if YOU can (if you want to) take the dog out regularly for some stimulation. It might be a nice bond for you and the dog, and the dog would surely appreciate it. and since you are there for hours anyway....
But trying to change your friend, that does not sound likely at all.
And hanging out there, given the way you feel about animals does not sound like it is doing you any good. I know that i personally could not hang out regularly in the same situation as you if i felt as you did....unless I felt that i was helping the situation.
I'd love to hear what other members feel about this.