My wife says he was friendly/apologetic at the side of the road but when l rang him later he had changed his tune a bit "how do l know the damage was not there before" he said.
He is stupid and it took one of his friends to point this out to him, at the side of the road he is unprepared, he has now had his friends and family tell him what to do so he probably realises that this could go in his favour. He will also probably be all to aware it would be easy for him to say your wife drove into him.
First thing I would do is record a conversation with him, phone him up in a quiet room and record it on another phone or on a PC etc. Use speaker phone. Phone him up and talk to him calmly, and get him to incriminate himself. Make a list of the points you need to get him to admit, including his name:
Hello is this Mr John Smith (yes)
I am Jim Smith you reversed into my wife
I just want to try and sort this all out (hopefully somewhere he will say the words you need).
Use different tactics, 'You know you reversed into her, why don't you make it easy on yourself?' Hopefully he will make a comment like 'you have no proof it was me'.
I know this would not hold up in a court, but you need to create some ammo you can use in a private fight here.
Once he has dropped himself in it suitably and continued to lie, you then inform him the conversation has been recorded in front of witnesses, you have him identifying himself on tape and admitting he did do it but he is going to try and get away with it and that you have dash cam footage which along with the recording you have now been forced to submit to your insurer.
Did he take your name and registration details? In other words could he commence a counter attack and have his insurance company make a claim from yours?
Are you sure there were no witnesses?
Is there CCTV in the area?