Why not take out an ad in the local newspaper stating the time, date etc of the accident and ask witnesses of the crime to contact you? That's how witnesses are found here for accidents. I can't believe that person drove off after the drive and didn't call in the license number... unless they lost the person.... but even then they should have notified police of the colour and type of car. However, it sounds like the person just didn't want to get involved and only said that they would help. Karma!!! It will get them both back in the end and it won't be pretty when it does.

"It is good to see ourselves as others see us. Try as we may, we are never
able to know ourselves fully as we are, especially the evil side of us.
This we can do only if we are not angry with our critics but will take in good
heart whatever they might have to say." - Ghandi