It's hard to explain because the chapters were from the POV of different characters, so where the book ends we are only seeing one person's POV.
Aisha found out about the affair and eventually realised it was with Connie, the teenage girl working part time at Aisha's vet surgery. I think in the book she sacks her.
She finds out because the teenage boy, Richie, tells Rosie and her husband that Aisha's husband, Hector, raped Connie.
Connie had lied to Richie about this, because Hector ended their affair.
Rosie and her husband force a big confrontation, under the guise of being concerned for Connie and Aisha but really to get their own back. Rosie is angry at Aisha because she let slip the information that allowed Harry to walk away from court, the husband has always hated Aisha and feels she is stuck up.
Connie lies again and says Richie is making it all up. He then goes along with this but Aisha realises the truth anyway and I think she sacks Connie.
I think Harry gets away with slapping Hugo, Rosie is humiliated in court because it comes out that she once abandoned Hugo and drove for three hours to stay at a hotel. I think she had PND at the time. This is the information Aisha had spoken about.
I think Hector and Aisha decide to give their marriage another try, after she goes abroad for a conference and also has an affair. I think Aisha is also forced to attend a party at Harry and his wife's house, which the family see as her finally taking their side over Rosie's. Aisha has struggled because she knows Harry is violent and once broke his wife's cheekbone. I don't think that comes out in court and Aisha feels guilty because she knows it could have changed the outcome of the case.
At the end of the book, the last chapter is Richie's story, he gets a date with another boy and they all go off to some festival thing and he makes up with Connie.
I may be confusing some little bits with the TV series, but I think that's the gist of the book.