You might take a look at the "Boss" panic buttons on some old games and sites. The idea is that you could switch quickly to something innocent looking if your boss came by (of course, you have to see them approaching first!).
Almost any hurried/frenzied action when the abuser comes in is going to arouse their suspicion. Consider designing the screens so that they look like general news or the like (from a distance), with no large titles or pictures giving away their purpose. This could often allow time for a "leisurely" exit from the site, without panic. If the panic button link takes the user to, say, a bona fide news site, so much the better (if it comes up fast enough).
As others have mentioned, if the abuser becomes suspicious (and many control freaks are), there's still a history trail on the browser that they can quickly find out where the user has been. Frankly, there's not much you can do, especially if the user goes into a frenzied panic mode trying to cover their tracks, and the abuser sees this. Even if the screen itself can be refreshed to something innocent after some timeout, the browser history will have the URL.
Consider advising users to position the computer so the abuser can't sneak up on them from behind, or to go to a library or other non-private place to use this site. If the screen content is dull looking from a distance (see above), it shouldn't attract interest from others.
Good luck with this!
