You can still live together. It's not that hard. If you can live separately, you can live together, and it's cheaper.
Divorces are often messy and stuff, and when the parents find new partners it can also easily get messy. But it's not like the parents have to give everything up for their kids either. She doesn't have to think of the new woman as her mother, but just thinking it's gross having her around is immature, especially since she's not given her a chance.
Yes, she's not an adult, but at 17 she should be sufficiently mature for interpersonal stuff like that. And if they want to play the "still a child" card, then it's entirely the father's decision that matters, because it's his responsibility. So if he thinks she should be mature enough, or that it will help her grow more mature, then that's it.
Or maybe I'm just talking from personal experience with both my parents finding others and not having my world collapse because of it. I still have one father and one mother, and the others are acquaintances, friends, secondary family members, or whatever.