Is the statement: "Artificial hymens are invading Arab markets for fifty riyals, especially for wives who have lost their virginity; so much for honor" - which was sent to the wife in jest - considered a false accusation [of unchastity]?

Defamation (Qadhf) is explicitly or implicitly accusing someone of adultery. What is mentioned in the statement is not explicit, and as long as it is said as a joke, it does not constitute defamation (Qadhf), because implicit statements and allusions do not become defamation (Qadhf) unless there is a clear indication. One should avoid such jokes and be keen on using good words.