Withdrawing money from a multiple name account?

In our mid 30’s and getting married in 3 months. My fiance had his mother handling his fiances in the past and now it’s becoming a problem because she doesn’t agree with the money we’re spending on the wedding. Now she refuses to hand over or change any of his banking information. Does anyone know if he can withdrawl the majority of his money to open a new account even though both his parents names are on the account with his? We feel this is the only way to solve the problem since she refuses to remove her name from HIS money.