You probably already know this but good and cheap don't usually go together in the audio world. Is your guitar electric or acoustic? If acoustic does it have pickups? Is he using a guitar amp? If using an amplifier you can mike the speaker cone or if it has outputs (1/4 in or XLR) you have that option as well. An SM57 will work as good as about any in that price range. A nice condenser microphone would be better but alot more money and it has to have a power source too. Just some basics, I don't know how knowledgeable your friend is with audio. There are forums on the net as well, you can ask questions and audio professionals (engineers) will give suggestions. I read prosoundweb.com regularly, if it's pro audio you can find it there. They have a complete section on recording. Have your friend GET EDUCATED FIRST, before you spend money. You will spend less money and get better results as well.