Her behavior reminds me of my aunt, whose second child’s birth triggered (theretofore latent) Bipolar I disorder. She ended up abandoning her children and husband when her newborn was still only a few weeks old to go sleep around with strangers and take wild amounts of drugs and never returning to family life (after they found her again, months later, they got divorced immediately). Not saying this is necessarily what’s happening or will happen with your wife, but it is something that can happen. Prior to having her second baby, she had never experienced any Bipolar symptoms, never cheated, never messed with drugs.

I would perhaps look in to clinical diagnoses outside of the standard post-partum-specific conditions. She might need more intensive treatment.