Some random thoughts...
I'm a little surprised by those posters who seem to think that The Salt Path would be Richard Osman's cup of tea because he wrote the Thursday Murder Club books. I like the Thursday Murder Club, but The Salt Path is definitely not my thing.
Regarding what is acceptable in a book like this, I know that there are always at least three truths - your truth, my truth and the truth. I don't expect a book like this to be 100% objectively truthful. The writer will embellish incidents to make them more interesting. When they record conversations, it is unlikely that they have recalled the exact words used. They may miss out events that put them in a bad light, or give a version of those events they do record that puts them in a better light than is justified. But I do expect a basic level of truth. I expect your truth, not something where important elements of your story are a complete fiction designed to manipulate the reader into feeling sympathetic for you.
As for how they thought they would get away with it, I am reminded of my second wife. She always claimed she hated liars and would never lie, but she was the biggest liar I have ever known. Often her lies were obvious. For example, for much of our marriage, she insisted that she had never wanted to marry me and that we had got engaged when I unexpectedly produced a ring and popped it on her finger (the reality is that we had chosen the engagement ring together that afternoon and agreed that we would get engaged during our evening meal so, at an appropriate point, I popped the question and she said yes). She could never answer the question as to why she didn't simply say no if her version was correct. Occasionally her version would revert to what actually happened, so she clearly knew the truth, but most of the time she would insist that the lie was correct and clearly expected everyone to believe her. The idea that anyone might question her version of events didn't seem to cross her mind. I could give many more examples. I wonder if SW is the same.