that's easy, it is implied that he was once married to her.
as for how to trigger it, simply sleep with her once, then inform him, but do that before ever telling him about Starmaker, meaning bypass drinking with him on the entrance every Friday, a different scene triggers, which also triggers him going to talk with Anna and both admitting and opening their relationship, which also implies Josef starts sleeping with other women.
basically a shortcut to keep Josef around.