Ugh... as said many times before, a computer without the restore CDs is like a car without keys -- would you buy a car from your friend and let him keep the keys?
No, you can't download them from anywhere. You either need the original CDs/DVDs that came with the computer, or your need to legally purchase a retail set of OS X install CDs/DVDs.
Kindly ask your friend to complete the computer purchase and include the original restore CDs/DVDs that came with the computer. After all, they're no good to him/her anymore, and the operating system license should travel along with the computer -- which means that the restore CDs/DVDs should, as well.
Eventually, you will find yourself in a situation where you cannot proceed without the operating system CDs/DVDs. You need them, as they include vital disk-checking software, password-resetting utilities, and reinstall capabilities. Your first quest in enjoying your new Mac should be legally acquiring some Mac OS X install CDs/DVDs.