I would focus on A1 German (beginner/elementary level). You can find a macro-level description of the competences at A1 here. Follow the external links for more thorough descriptions of the skills required at A1.
If you studied German for 6 years, where does “fluent” fit in the descriptors? It’s one of those really vague words that I hate as a language teacher, as it’s a relative term rather than a reference term.
Your level when you stopped studying would be, I suspect, at around high B1/low B2, provided you studied throughout secondary education and did not participate in immersion programs. How much of it do you really remember; how much is dormant? How much have you lost? I suspect that you will have a very spiky competence – you might find some stuff comes flooding back, while other stuff is totally ‘new’ to you again.
Taking a very simple, transactional focus in your conversation about concrete things – requesting food or borrowing something would be the simplest way to “win” the bet, but I’d agree on the criteria of success first.