If she has Full passive ROM (i.e.no capsular restriction), but cannot actively move through full range you have to suspect a muscle dysfunction.
How old is this patient? In older age groups, with inability to abduct and particularly if combined with night pain and weakness of abduction and external rotation a Wikipedia reference-linkrotator cuff tear should be suspected.

This paper will describe some of the tests that you can use to diagnose such tears.
Tennent TD, Beach WR, Meyers JF. A review of the special tests associated with shoulder examination. Part I: the rotator cuff tests.Am J Sports Med. 2003 Jan-Feb;31(1):154-60.

If it is pain alone limiting the movement, and you have ruled out a neural/neck cause, then a local steroid or even local anaesthetic injection will help clarify whether the muscles are pain inhibited or actually torn.