Speaking and communication are two different things as both have different concerned areas in the brain.
First of all identify what is the problem.
Is it that her speech is slurred OR there is difficulty in pronunciation of certain words OR there is a problem only in speaking i.e. what she wants to express she is not able to through speech but she can write well.
Next Steps
A complete description of above speech characteristics will help to diagnose what is the problem. Accordingly the treatment can be initiated. If a speech therapy is needed that will be recommended.