Jen Harley's attorney tells us ... the Clark County District Attorney's Office has declined to prosecute her for domestic battery and the case has been dropped. Law enforcement sources say Ronnie would not cooperate so, without a victim, the case couldn't go forward.

We broke the story ... Harley was arrested in Vegas back in May after a New Year's Eve incident with the 'Jersey Shore' star. Ronnie filed a battery report against Harley, claiming she attacked him inside a club. We obtained video showing Ronnie leaving the club with a bloody face.

Harley claims when she returned home later that night the place was ransacked, she believes at the hands of Ronnie. She accused Ortiz-Magro of smashing valuables, including a vase and a flat-screen TV.

He was also caught on camera smashing the security cam at her front door back in December.

Despite all the turmoil ... the 2 were seen taking a trip to Florida together last month with their daughter, though they didn't look too thrilled about it.