The husband confessed!
Nancy and Drew had made up but he was still unsure.Being an immunologist he knew the perfect way to test her fidelity at the upcoming xmas party. He invited the boyfriend out for lunch pretending to want to put everything behind them. At lunch he slipped brazil nuts into the boyfriend’s salad. They left each other on good terms.
At the party that night, alcohol flowed and well, things happened.
The husband is still upset but Nancy did cheat on him so he feels it is her fault.
Note: this mystery was based on a real case where a woman became ill after reacting to Brazil nuts in her partner’s semen.