Please don't tell her how you feel @Salvcx It won't help! My friend's DD recently left her partner (together for 13 years/married for 3, met at 18,) for another man, and my friend is upset because her DD is acting like her ex didn't exist. (And she cared about him a lot/thought of him as a son.) It's like he's been deleted from her life!
Every photo gone off Facebook and Instagram of them both, her DD's surname changed back even though they're not divorced yet, moved into a 2 bed house within 1 week of ending it, (clearly was planned for a while,) and she moved in the new man a few weeks later.
They are off to Morocco together next week for 2 weeks. Such a lovebomb and such an intense relationship so quickly. They only met in January (apparently!) The DD said her ex and her had nothing in common, they met in their teens and she 'settled' as he was her first boyfriend, and she thought this is as good as it gets... They are now in their early 30s, and he bores her, and is obsessed with sport/cycling/football...
She has moved on/grown up between 18 and 31, but he hasn't. He wouldn't even move more than a mile away from his mum and dad. (And he - and sometimes her too - went to their house twice a week, and his mum and dad popped around twice a week too... So he saw them FOUR times a week, and she saw them three times a week sometimes. (At least twice every week.) My friend only got to see her DD and her DD's husband twice monthly! SAnd one of them was when she visited them. They only came once a month! And she lives/lived 10 miles away from them.) And they always went to see his gran every Wednesday for 'tea.' Whether my friend's DD wanted to or not.
She wanted to move to the city 20 miles away. She said her life was on hold for 13 years. He didn't want children, and she did, and the had sex once every 3-4 months. (Why do I know so much? Because she is very open about it all.)
tl;dr Your daughter will be much happier not sticking with someone she doesn't love/care about any longer. Better than not realising she's wasting her time (and her life) for 13 years, at the age of 31 ... Like my friend's daughter. She does seem much happier with the new man, but it has moved verrrrry fast! Oh, and my friend sees her DD more often now - like weekly... (She visits her every 2 weeks, and her daughter visits in between, sometimes with her new man, sometimes not.)