Absolutely justified. This girl doesn't know the meaning of the word "respect," and she's made it clear she has no intention of learning it.
A 19 year old is legally an adult. If she doesn't have a place to stay because she is unwilling to be respectful toward her dad's girlfriend, that is HER FAULT.
Period. End of story.
It is unfortunate that the girl's primary goal is to get her dad to break up this relationship.
Dad should NOT allow that to happen.
If he does, he will suffer the same horrid behavior from his daughter repeatedly.