To your specific question in terms of code: A good idea would be to use the facade pattern to have a single point of access to the model from the controllers, so you can always call the facade and don't have to think about how his model logic will deal with the request internally, or which classes he will use. Furthermore, if MVC is done correctly the model, view and controllers will be swappable (as in, you could develop the view/controllers in javaFX but someone else can just use the same model and write android views/activities to call the logic). Meaning that you should not tell him what to provide from a controller point of view, but you should have your controller work with what the model provides. Ofcourse, the model has to provide all the necesarry logic so here are some tips on how to achieve this in general.
To your question in general: If it's just you and a friend there are several things you can do to improve smooth development. Communication is important here, make sure you have predefined what you want to create and discuss some aspects of how this might be done.
Maybe create some user stories (http://en.wikipedia.org/wiki/User_story) or create a GUI mockup (each create a mockup, or you just create one) and discuss it. This will make you know what is required from a user point of view in the GUI, and he will see how your GUI handles this and will know which data will have to be provided by the model.
In addition you could try pair-programming, so he gets a feel for the view/controller code and you get a feel for the model code. Ofcourse you're not strictly developing apart anymore then, but rather together on some parts. Anyway, as it's only a two-man project and you both get along, this might be a fun and enriching experience.
Anyway, if you run into a problem where you think "Frick, I need this data but the facade doesn't expose it" you can just tell him that you need that bit of data and he can create a method in the facade that provides this. Sometimes requirements change or you have to do something you didn't think of before. That's software development though ^^