There is a paragraph on pp. 73-74 talking to the unfaithful partner, but really about chemistry in relationships in general.
It says that high chemistry between two people may be triggered by an unhealthy model of love. Low chemistry "may mean that you've chosen a partner who allows you to experience yourself in a more positive, more fulfilling way than you did as a child."
Anyone else read this and scratch their head? I'm kind of rewording it in my mind as advice to not use feelings of excitement as a judge of a good relationship, but the word "chemistry" makes me think more of general compatibility.