Need advice on joining an early startup

A person in my network offered me to work with them on a product. In my eyes, the idea and market they're targeting is a good bet.

They've offered me to come on as an engineer and work with them. A lot of the work of the MVP has been done but we need to make a few changes to get it ready for launch. From my initial idea, this will also take quite some time.

Now the thing is, I'm ready to onboard with them but when I asked them to give me access to the code so I can start working with it. They refused to give it to me fully and that seemed like a red flag to me. In this case, should I work with them?

Maybe they don't trust me fully but then why would they offer me to work with them at all? I've known this person for quite some time and have helped them with a few other things.

Should I communicate my unease? I would like to give them the benefit of the doubt.