Jennifer should be very consistent in retelling her story. If she keeps changing the details, her mom will continue to doubt. So, she needs to make sure she tells the exact same story every time.
She could ask a reliable third party to vouch for her. If there was someone else involved in the story, like a friend who was with her at the time, having that person confirm what she says might make her mom believe her.
She can provide evidence. For example, if she says she went to an event, she can show photos or a ticket stub.