He just needed to promise her that he will forget all she did to Jasmine in the past to get the lady to start talking.
Samantha was the one who introduced the man selling poison to Jane when the two of them had previously planned to poison Jasmine. That plan never held since she changed her mind at the last minute and got rid of the poison.
Even though she hated Jasmine a whole lot she didn't have the heart to kill her. She was once her friend after all so how could she bring herself to do such a thing? If not for her conscience but her future, she had backed out of the plan at the last minute.
The poison guy and she were very close so following her orders he had called her when Jane got in contact with him. And that is how she knew Jane's plans.
It was during this period that having installed listening devices to Jane's favourite car and handbag came in handy.
