Try to get the birth certificate corrected by contacting the Oklahoma State Department of Health Vital Records Service. See link below.
However, if she has a birth certificate, even if it is currently incorrect, you still may be able to use it in order to obtain the marriage license from the court clerk. An amendment can then be done later.
The answer given does not imply that an attorney-client relationship has been established and your best course of action is to have legal representation in this matter.