The letter of the EULA says the friend "may not rent, lease, lend, sell, redistribute or sublicense the Apple Software". That agreement binds the friend, though, not you.
If you believe you acquired a license to Snow Leopard when you bought the computer - i.e., the former owner had truly lost the install disks; not just retained them to reuse them - then functionally, you have the right to re-install Snow Leopard. A bill of sale from the former owner would tend to confirm this. Technically you're supposed to have your own disk(s) to do it but I can't imagine Apple caring. EULAs are generally intended to prevent piracy, not to block legitimate use in the face of technical difficulties.
Obviously, this is my personal opinion and I doubt there is language in the EULA to support it, but I would have no concerns that were Apple to discover such a re-installation, that they would care about it. (That's not to say that asking one of their lawyers for permission mightn't get you a formal refusal; lawyers are paid to hold the letter of the [law, agreement, contract, what have you], not to interpret it [unless there's some money to be made? -- but maybe that's just my cynicism!]).
Bottom line: if I got stuck, I'd go ahead and do it with a clear conscience. But if it were me, thinking ahead about it as you are, then for only 30 bucks, I'd just buy a retail install disk right now today, and rest securely in the knowledge that I'm prepared to maintain or repair my OS, no matter when, where, or what.