I'm the son in this situation but it is still somewhat relevant.
After 15 years of what seemed like a happy marriage, when I was about 10, my mum got bored and had an affair with an ex-boyfriend of hers. Then she swiftly followed it up with another affair. My dad found out after about a year and had a few affairs of his own as well as going to a few prostitutes.
They argued constantly in front of me and my 3 older siblings. My mother would often get violent, hitting and kicking my dad until 3 of her children were on the floor crying and begging her to stop and see what she was doing. He never fought back physically but his words were barbed with venom whenever he spoke to her.
To us kids they both did the same routine. "Oh I still love your father/mother very much and we will both always love you." Being the youngest of 4 I was very much protected by my 2 sisters (my brother was away at boarding school. He still knew the jist of it but my sisters both knew more) and they told me what they thought I needed to know. We are all very tight-knit so I knew most of it, just not every detail and I'm glad I didn't at the time.
The affairs, arguments and tears continued. My dad had a flat in London about 2 hours away so he would mainly stay there during the week. At weekends he would come home to suffer 48 hours of solid abuse, giving less than he got but still not shying down. I thought he came back to try and keep up some semblance of a happy marriage but I found out later (a few years later I was looking for porn on his laptop and stumbled across his journal) that he came back every weekend because he was so sure that despite everything this was the woman he loved.
Right before I turned 14 I went to boarding school as well. Very aware that things weren't getting better despite a year and a half of marriage counselling, I was nervous about what may come. One of my sisters was leaving to university and I thought that both of my parents had only stayed in it so long for the children's sake. During the christmas holidays the 6 of us were sat down and my father made te announcement that he was moving out. He had rented a beautiful cottage in a village a few miles away. The lease was only 6 months whilst he looked for somewhere more permanent. I broke down in tears as did one of my sisters. The other two siblings remained stony faced. They had been told beforehand by my dad. They spent the rest of the weeks reassuring us.
I went back to school in January. All my worst fears had come true. At the start of March I got a phonecall from my mum at 2am. She was drunk, crying about what a mistake she had made and how sorry she was. 4 weeks later my dad cancelled the rest of his lease and moved back in. Slowly, bit by bit they started getting better.
10 years on my friends regularly say how nice it is to see two older people who are so obviously in love with each other. There is the odd argument over petty things nothing major. Everyone is happy.
Basically. Second chances can work. People can make mistakes, major mistakes and still find a way back.