If you're short on cash, you should probably be looking for a client that will actually pay their bills. Tell your "friend" that your work on the project will stop until the work that's been completed has been paid for.

What else you can legally do depends on the terms of your contract. To protect yourself, you probably shouldn't take any action until you've talked with a lawyer. You don't have to bring an expensive lawsuit against them - just pay the lawyer for an hour to review the contract and advise you of your legal rights and responsibilities.