I'll echo the already-posted sentiments of a few.
I'm not sure how your administration at your school works, but I'm sure it doesn't differ too much from mine. If you tell your prof, he can't do ANYTHING about it - he didn't see anything and if he asks the student if she did cheat, she will deny it at the very least. At worst, she can file an academic complaint saying the accusation "interrupted her academic progress" or some other BS excuse which the Faculty and/or university administration will have to take VERY seriously.
The benefit of telling your professor what you saw or suspect is that he/she can keep an eye out the next time. The prof will more than likely not say anything to the student in question as it will result in either the situation above, or the student won't cheat anymore. What the prof will do, however, is keep a keen eye on that student the next time and try to get other eye witnesses of her future behavior, if possible.
Trust me, having gone through a few academic misconduct proceedings, the set-up is very much in favor of the student unless the prof has TONS of eyewitness accounts in writing. So, you just saying to the prof "this person cheated, I saw them" is kinda useless in the immediate term. She'd have to be really dumb to do it again, but if she does, at least the prof can watch/prepare appropriately for it.