Why didn't Tiana just ask Charlotte for money to buy the restaurant?
She could have asked Charlotte if she could borrow some money.... I'm sure Charlotte would be willing to help, and certainly Big Daddy wouldn't say no to Charlotte.
Now before you say something like "Tiana wanted to work hard to earn everything she had; she wouldn't have accepted it if Charlotte had just handed her the money." Point taken, but she certainly seemed willing to just get it from Naveen, and when she found out that his plan to become rich was to marry Charlotte, she still was willing to go along with his plan. So basically, she would just be getting the money from Charlotte anyway?