In the absence of proof, practically most of the property transactions at present are done through this method only.
The question of tax implications never arise as because in the eyes of law the cash paid in excess of registration value never comes in view.
You should be aware of this. Do not worry.
And it should be beneficial for your friend as it will reduce the Stamp paper fees of your friend.
But as far as long term benefit is concerned Property should be registered with full price as it is agreed between the parties. It is so because the more the registration value of the property the more loan will be sanctioned by the bank in future.
With Regards,
Kalpesh