Who's car is that taking my girlfriend home?

I went to pick my girlfriend up from work as a surprise, but she got into someone else's car. She returned home late. This wouldn't have been suspicious had she not told me trains where late! I didn't get a very good look at the person, but it did look a little like a friend of mine. All I got really was the car make and last 3 numbers on number plate. It's clear the relationship is over, but I need to be sure who it is, because if it's who I think it is I need to cut them out of my life, as I don't need friends I can't trust. Is there a way I can find out who the car belongs to? Thankyou.