What you both need to do is to work out exactly what work needs to be done. How much footage is there, how much has to be gone through to pick out the shots, and what subsequently has to be done once you’ve chosen them. Don’t forget meetings, review sessions, and hours spent getting things “just right.” Consider the music and sound, as well. Explicitly deal with who owns the copyright to the finished work. Make sure they can’t use it until they’ve paid-up, and make sure also that you can use it yourself as a promotional piece whether-or-not the picture gets released.
Then … write it all down, attaching your price to this detailed estimate, make two copies, and both of you sign it. Each of you keeps a copy bearing your two original signatures.
As work progresses, keep a detailed daily work-log including actual time-stamps. Write down exactly what you did and when you did it, day by day. Be truthful and meticulous. Submit a regular progress report to the client. Your contract should have stipulated that he or she will make regular “progress payments” to you.
There are plenty of books, most notably those by Herman Holtz, on “consulting contracts” and other forms of working agreements. Both of you need to be familiar with them.
The purpose of this advice is not that you should be “confrontational,” but let’s face it, no one expects trouble when they’re starting out. You must set expectations, look for contingencies, and address these issues b-e-f-o-r-e you set sail. This is “good business practice,” and it means that you are professional. Those things get noticed and remembered.
P.S.: 90% of all “gonna be’s” aren’t “gonna be.” It’s just the way it is. Dreams are cheap; film is not.