I find that the majority of you all are focusing on 'grr I feel so sad, angry, hateful towards the daughter' your missing the resignation she shows in giving up her unfruitful love for her friend and chooses to root for her friend's love with her own mother regardless of the cheating going on. At least that's what I got out of the ending. Bittersweet. I'm rooting for you type of vibe. Cheating regardless even if it's my mom. I want you to be happy.
One fucked up moral I've seen in real-life and fictional is that the happiness of one person from a bystander's (think of the daughter wanting her friend to be happy overall and she supports her as an example) point of view is more important than if it's morally wrong or not.
Some people take the definition Love is Love to far yes they write it in ways that it can be understood. That in a twisted way it's okay. In a twisted way, mind you. I'm not giving this angle the society norm type of way 'cheating is bad no matter what' type.
This angle is more of 'yes cheating is bad but a person's happiness isn't something to be ignored and matters overall'
Sometimes to be with someone you truly Love you have to go through the rough path that is cheating. It's fucked up Morally yes, but life just isn't simple as people say or demonstrate.
People make life complicated by being unable to rein in their emotions and do the right thing. It takes a lot to be courageous, brave or to simply be a better person. Unfortunately.