I didn't think there were any porn apps so why would you lock one?
Lockdown Pro and Locktopus don't work on my iPad 2 running 5.0.1.... keeps crashing springboard. FolderLock works, but you can't lock apps... only folders.
Maybe he wants to lend his iPad to someone else, like a friend or family member, but doesn't want them to view his facebook page or twitter account?
Back to the topic, no, I've tried all, but like you said, nothing works, yet.
Have you tried Locktopus?