It seems obvious, but perhaps you should ask your bosses what they want you to ask or find out about this person? At best, your role here is to screen the person for your bosses - at worst, which seems to be the case here, it's to waste everyone's time. If I were interviewing for a senior position and I was asked to get lunch with a junior person with no clear agenda, I'd be concerned about the lack of seriousness of the organization.