Interesting post. I can't comment on legal obligations, since I'm not a lawyer (much less a Montana lawyer), but if simply shaking an infant is reportable in your state, then it seems to me that you have to do it. Several things come to mind:

1. Most important, if you believe it is likely that an infant is in danger, I wouldn't be very concerned about my therapeutic relationship with the non-shaking parent. For goodness' sake, protect the infant. And don't wait for absolute proof; your good-faith belief that injury seems likely is enough.

2. But: Do you have reason to believe that the shaking is serious? I understand the dangers of shaking a child too vigorously; however, do you believe the shaking is in anger rather than gentle play, and rises to the level of danger (and thus you must do something about it). You may also be dealing with a custody battle (or a future one), a mother who is misrepresenting her husband for some other reason, or a patient who doesn't preceive the "shaking" accurately. In any event, though, protecting the infant from potential significant danger is the top priority. CPS staff -- not you -- are the professionals at determining the facts and acting properly on them. You probably don't have access to all the relevant facts, nor are you in a position to act on them. (One must hope, of course, that CPS is a fair, competent, and reasonably-funded agency in your area; that's not always the case, but they usually try hard.)

3. Regardless of whether or not a patient signs an acknowledgement that the therapist must report things, etc., the therapist is still bound -- ethically in my view, and very probably legally -- to do what is right. It's nice that you went over the initial form with her, but it should not affect what you do about the child (or what you have a duty to do).

4. Finally, a Web forum is not the place to get legal advice. I suggest you contact a (Montana) lawyer if you are not sure about your legal obligations, and a senior counselor with family experience if you're not sure about the practical and ethical ones.