I think that if she wants to do it, she can, because pageants are all about looks, not how you obtained them.
Essentially, it would be just plain wrong to not let her be in the pageant. If her ID and birth certificate and everything else say female, she is female. Obviously a therapist would agree that she's trans, and therefore she would be female.
If anything, I believe that anyone who identifies as female should be able to compete in these types of beauty contests.
