Help this young person to consider if their mentor is the right fit a few times a year. What could be improved? What works? If this seems to work well, tell them to speak to their mentor about continuing the relationship.
Ensure they are armed with some good strategies for example, to be a good listener, keep eyes and ears open and always be willing to help. If it does not seem like a good fit, help them evaluate what they've learned and begin exploring other options for another internship following these same steps.