What single subject is she interested in? I believe multiple subject is K-6 so she could teach 6th grade with a multiple subject credential.
Did you compare the classes needed for multiple subject to single subject? It does seem as if it would be difficult to do both.
Also, my understanding is that if a district hires you and you have multiple credentials, you can be assigned to teach anything that you are credentialed in. So if she had multiple subjects plus science, she could be asked to teach anything from kindergarten to AP physics.