Username or EmailPassword
The question seemed to indicate that the one person had moved from 10.6 to 10.7 installed on the machine. This providing the 10.6 install DVD for the second person.
An issue to be aware of here is that 10.7 is likely considered an upgrade not a fresh install. The machine is being upgraded from 10.6 to 10.7 and 10.7 was sold at a low cost based on the expectation that the machine was already running a legal 10.6 install. They may make an argument that both 10.6 and 10.7 install media belong to the one (now) 10.7 machine rather than the upgrade freeing 10.6 up for install on another machine.
I'm not deeply familair with Apple licensing policy though. That's just the issue that came to mind based on the age old Microsoft trick of "upgrade" installs requiring validation of the prior "full install" version it was meant to replace. It's a friggin pain too because now you have to keep the install media for the prior version encase you have to prove that you are upgrading during a re-install of the newer version.