But seeing as how he's replacing it, the front screen part would probably cost a fortune.
The entire front assembly is sold in once piece. For a G1 it's about $50, for second and third generation ipod touches it's about $70. They don't have any for the 4G. The LCD screen itself is about the same price as well, but it looks like he's just cracked the glass (which is really common). Whether that's a "fortune" or just "an unfortunate extra expense" for you really depends on how much income you have.
Of course, if you don't have the tools you've got to buy them as well.