I would try to start with just baby steps with her.

Take her outside by herself and leave your boy inside. Maybe intice her with food, or a treat, or something to distract her to get her outside and then keep her entertained and her mind off of the boy that she's missing.

Just for short periods of time and try to lenghten them out.

All happy happy joy joy while she is away from him.

Sounds like she's in love BIG TIME