Yeah, the problem is that this Anna is not a corruption story, she is a slut who had normalized many things that are not normal, who played the "good mother" acting the years required to get a valid man to become the father, and now she is returning to her slut mode. Pretty common nowadays with women waiting until mid 30s to search for a husband when their biological clock is ticking, and this is why clever men search for their slut past and red flag old slut women, they never recover at all, they are wasted.
Its not only that she is sexually unfaithful. Its worse, because at the end of the day, if you are in a 40 years marriage from 35 to 75, if one day you had pleasure sex with no connection, is not that big of a deal, you just don't talk about it. The problem is that your husband is supposed to be your life partner, and her flirting with his best friend or letting it happen, is several levels down in her loyalty if that exist at all.
First of all, not telling his husband that his "best friend" is not his friend, he is playing behind his back. Second, she is the partner in crime of that betrayal. Sex or not (sex itself is highly overrated), she is not just sexually cheating, she is a truly bad person by playing this game behind his back and not letting him know. I know this game is played by young and naive guys, and all it takes for them is to show a "damsel in distress" so they can play "captain save a hoe" or the "white knight", but she is truly bad person, playing truly dark and twisted games, AND SHE KNOWS.
The only thing left in this novel is watching her perform the cheating, then it's dead because not a single one of this characters are truly likeable.