Am I allowed to install Snow Leopard on my MBP using someone elses install disc? I bought a 2009 Macbook Pro 13" used from someone and it doesn't have the install disc for Snow Leopard which is currently installed.  If I have to restore the computer someday can I use a friends Snow Leopard Install Disc? 

I'm not sure how the licensing works.  All I know is right now I have a legit version of Snow Leopard on the computer. The install disc's are like $50 and I don't want to buy one unless I have to.
Yes, if it's an install disc that was either a boxed copy, or an up-to-date CD, no if it was just the disc that came with the machine unless the model number of the machine is the exact same one.

There are no license keys or otherwise.

You may not be able to install bundled versions of iLife as they are often locked to the machine it was bought from (but even this is get-roundable by showing package contents and using the individual packages rather than the wrapper installer), but the OS is usually fine.

I think the only install media that isn't usable across a number of systems are the Lion USB installers which I believe are tied down at serial number granularity.

Frankly, if you are worried about licensing, just upgrade to Lion for $30.