How do you download a purchased copy of OSX Lion as a file?

A friend wants to give her dad a pre-2012 iMac, but wants to wipe it ahead of time. It rejects the media that came with the machine (when it tries to boot to the DVD, the screen goes gray and thee beeps repeat... this seems to indicate a RAM problem, but I know that's not true, as it boots normally) I'm trying to download OSX Lion, which I purchased. I want to download it as a file. There is no option to do that. Help?