Cancer cells do not, as a rule, make a promise to you that they will cherish you and look out for you and that you can therefore let down your guard and trust them. The shock of discovering cancer is severe, but it is not compounded by a sense of betrayal, which is searingly painful in a way that physical illness is not. If you don't believe that, then I can only assume that you haven't had to experience these miseries yourself.
It is certainly NOT a given that a man will cheat; the gloomy statistic that 60% of marriages will experience infidelity implies that 40% of them WON'T. And many of those 60% failures will count as infidelity because a new relationship is started before the sick marriage is legally ended - i.e. it's closer to serial monogamy than to infidelity. The idea that men are constitutionally incapable of being faithful to one women is a self-fulfilling prophecy that does us few favours as a society.
I'm not interested in craft or DB, but her weight loss is typically what happens to spouses who discover they've been cheated on when they had no prior suspicions. It's called the Infidelity Diet - an inability to hold down anything but pills and coffee, and a constantly thudding heart and racing metabolism. She'll put it back on, eventually.
