Final answer:
Lisa's mother's specific advice before boarding the Kindertransport is not mentioned, but it would likely include being brave and remembering her family, as with many children in that situation.
Step-by-step explanation:
The advice that Lisa's mother gave her before she boarded the Kindertransport is not explicitly detailed in the given references. However, the Kindertransport was a rescue effort that took place during the prelude to World War II where Jewish children were sent to Britain to escape Nazi persecution.
The children, often separated from their families with the hope of being reunited after the crisis, carried with them not only their physical belongings but also emotional advice and wishes from their parents. Given the context and the nature of the situation, it can be inferred that Lisa's mother, like many others, would have advised her to be brave, stay safe, and remember her family and heritage, with the poignant hope that they would be reunited in the future.