Lost Gmail password from auto-password generator?

Sounds like a tricky situation because you didn't tell me which password generator your friend used, but odds are good that whatever he ran, it likely has some sort of internal database of encrypted passwords.

If it's really well behaved, it might even be adding the password information to the existing account and password storage system on your Mac OS X system: Keychain.

In fact, let's start there. While you might be familiar with Keychain as the utility that applications like Safari use to store passwords, you might not realize that it's actually a separate application you can launch. To find it, go to Applications --> Utilities and look for "Keychain Access"

The opening window looks like this:

Scroll down (or search) and see if you can find the account information in question. Once you do, select it and choose File --> Get Info (or just press Cmd-I) and it'll show you more details:

Check "Show password" and it'll ask you for your system password:

If you enter it correctly, you'll now have the password for that account revealed. Helpful!

If you can't find it there, try the password generating application itself. For example, I use 1Password for account management within the Web browser and it too has the ability to show you otherwise encrypted passwords, as shown here:

Again, find the specific entry you're interested in and choose it. Immediately, the lower pane will fill with account details:

If the password isn't shown, simply click on "Edit" and it'll reveal it.

Hopefully these will give you and your pal some ideas about how to recover the password your application assigned to his Gmail account. Good luck!