If they cannot obtain financing from a bank, that has a proper system in place for vetting them, and would be less affected by an individual loan default, how is it a good idea for you to do it?
(This from someone that has financed cars for three* different individuals - friends and family - over the years. And I didn't lose a penny, but I still strongly recommend against it. I cannot even begin to imagine doing that with strangers. As a landlord, I use MySmartMove.com to run background/credit checks, and that's just to help ensure I get monthly payments and don't have to kick people out. With a car, once the title is in their name, you're not going to have a lot of recourse.)
(*Actually more like five if you count ex-girlfriends. Don't ask. Actually, I don't care. I have no shame. I'll tell you the stories.)