The joke is the absurd and funny statement, “ladies, my wife is single (and you should date her (implied))”. Basically they are best friends who broke up and now they are supporting the other dating by joking absurdity of the situation.

Comics don’t have to have a joke, they can just tell a story. This is a nice story of two people who care for and support each other in spite of discovering their selves and life goals don’t sync with continuing the original relationship. They manage to break up the romance with each feeling freed rather than dumped, so they can continue to be friends. Hopefully their eventual new partners will appreciate this.

The link has extra panels and a comment from the artists that explains it more. This isn’t a joke so much as it is explaining their situation in a joking manner.