Upgrading from snow leopard to...?

If your friend gave you a MacBook Pro with Retina display then all you need to do is to boot OS X Internet Recovery, erase the startup volume and reinstall its originally installed OS. No Apple ID will be required, and you don't have to pay for anything.

Read OS X: About OS X Recovery - Apple Support

Don't purchase a Snow Leopard DVD. Not only will it not work, any Retina MBP will be able to use OS X Internet Recovery. No discs are required.

If in fact you have an earlier model MBP, not the one shown in your profile information, then write back for instructions.

Retina MBPs do not have internal optical disc drives. Boot OS X Internet Recovery anyway and determine if you can reinstall its originally installed OS without an Apple ID.

Early 2011 MBPs shipped with Snow Leopard, later MBPs did not, and it's those that will not require an Apple ID. If it shipped with Snow Leopard, then yes you will need to purchase that disk so that you can install Snow Leopard.

If it shipped with Snow Leopard or earlier version of OS X, your friend would also have had an original System Install DVD included with that Mac, but if she lost it you can purchase a replacement or the retail Snow Leopard DVD.

Here are the comprehensive instructions for taking ownership of a previously configured Mac:

Refer to What to do before selling or giving away your Mac

1. If you enabled FileVault, disable it in System Preferences > Security & Privacy.
2. "Deauthorize" your iTunes account. Same for Audible if you have one.
3. System Preferences > iCloud > de-select "Back to My Mac" and "Find my Mac".
4. Sign out of iCloud. Select "Delete from Mac" when it appears.

Next: Remove all your personal information by completely erasing the Mac's internal storage.

1. If your Mac shipped with a grey System Install DVD, start your Mac with that disc inserted in the optical drive while holding the c key to boot from it instead of its internal volume, which should be erased before selling it.
2. If your Mac did not ship with discs, boot OS X Internet Recovery:
   • using three fingers press and hold the following keys: ⌘(command), option, and R.
   • With a fourth finger press the power button to turn on the Mac.
   • Keep the other three fingers where they are until you see the "spinning globe" icon.
   • This method forces the Mac to download its originally installed OS from Apple's servers, which will not require an Apple ID to install.
3. Remove any Open Firmware password if you created one: select Firmware Password Utility from the Utilities menu and remove it.
4. Select Disk Utility from the Utilities menu.
5. Remove any partitions you may have created.
6. Select the Mac's hard disk icon, then select the "Erase" tab.
7. Select the "Security Options" button and erase the disk.
   • The more "securely" you erase the disk, the longer it will take.
   • The fastest method is sufficient since all but the most expensive techniques and equipment will be able to recover securely erased data.
8. When it finishes, quit Disk Utility.
9. Select Install Mac OS X from the Utilities menu.
   • An Apple ID will not be required. If a prompt for an Apple ID appears, return to Step 5.
   • Do not create any user accounts.
10. When it finishes, shut down the computer.
11. If you want to install the bundled apps that were included with your Mac, restart by using your Applications DVD if one was included, and install the bundled apps.

    Apps bundled with newer Macs that shipped without discs cannot be transferred. Its new owner must purchase them from the Mac App Store using his or her own Apple ID.

If the Mac is being sold to someone outside the family consider the following additional information:

1. System Install DVDs that came with your Mac should remain with it forever, and must be included with the sale.
2. Consider including your AppleCare certificate if you bought it, printed documentation, even the box if you still have it.

   AppleCare stays with the equipment and is transferable.

3. Execute a bill of sale showing the Mac's serial number.
4. Once no longer in your possession, remove the Mac from your devices in My Support Profile.