"Decedent" is mostly used in legal writing to mean a dead person. Our mutual pleasures are now decedents. A figurative speech to mean that we don't like each other anymore.

The word 'decedent' is sometimes used as a noun in American English to mean a person who has died. The author could be saying that two people felt the pleasure between them had died. If that is what the author intends, it's an ugly piece of writing!