My friend installed ubuntu via wubi. He says when i select Ubuntu from grub menu, i see Ubuntu logo. One of the orange points turns into white and then there is empty black screen. What's wrong?
This question reads as a graphics issue on boot - possibly an incompatibility between proprietary graphics drivers and KMS - or possibly a problem with the default opensource graphics drivers.
There is not any detail in the question as to what graphics the 'friend' has in his computer, so try the following generic advice:
Add a grub boot option i.e. nomodeset for nvidia/ati or either i915.modeset=0 or i915.modeset=1 for intel based graphics.
See this question and answer on how to add those boot options and it also gives advice on how to fix it permanently in grub.