If it appears that there is a situation like that, i would IMMEDIATELY draw my peice, and use my other hand for the cell phone. After briefly calling either 911 or your local PDs emergency number (which is a lot better than the former), i guess it would be dependant on how i felt that day/night.
If i were feeling energetic, id run to the man, and give him a hollywood line with the muzzle to his head.
If i were in a normal/sluggish mood, id approach him while yelling questions at him. "What the (insert your favorite word here) are you doing, jack?"
"Are you raping her? Is that whats happening?"
"Get the (insert) off of her."
"Dont move. Dont move, man."
Its best at this point to check to see if the girl is okay. You'll know right then and there if it was consentual or not. If shes crying, whatever, assume the worst. If not, lighten up a BIT, while badgering the man with questions.
Your job isnt to be the judge or jury, but rather to keep him covered and stalled until the police get there. At that point, its in their hands.