Wrong Door by Bunty Avieson

It was the talk of the wake. The woman in the red dress. Everyone at the service wondered. Who was that woman?

Pete's dead and Gwennie's life will never be the same. How could Pete, a young, fit man, leave her now? Their lives together were only just beginning. And pneumonia? It was insane, unthinkable, unbearable.
Somehow she struggles through the funeral in a daze, and the mysterious mourner in the tight-fitting red dress barely registers in her consciousness.

It's only later, when spotting a discrepancy in Pete's tax records, that she begins to wonder. Who was that woman?