Joe and Beck date on and off for a few months, but after Peach's death, they really connect and seem like they're in a good place (from Joe's perspective). But then he figures out that the whole time they've been together, she's actually been sleeping with her therapist, Dr. Nicky (played by John Stamos).
Joe goes to get treated by Dr. Nicky himself, and through that connection he's able to steal some of the therapist's files on Beck. Nicky describes her as deeply narcissistic, highly sexualized, a pathological liar, and plagued by daddy issues (all true — Beck isn't a crazy killer, but she's also kind of the worst). Although Nicky — who is separated from his wife — starts off their sessions knowing full well that Beck is desperate for attention, she succeeds in seducing him and they start sleeping together regularly.
He buys her lots of expensive gifts and tells her he's going to leave his wife and kids for her. Unfortunately for Dr. Nicky, Beck acts out the way she always does: by ghosting him. He's pissed, and Beck runs back into Joe's arms.
Image Source: Lifetime