That's also my belief. He doesn't want a simple menu, he wants to show/hide some whole categories. I think this is something similar to the navbar menu I made here:
http://aelyon.eg2.fr/forum.php (click on the central button and/or on the calendar button).
The thing is that he will have to edit the index_bot.tpl file and change the part where the forums & cats are defined to some div with a tab-header and a content hidden which would appear only when clicking on the tab. If this is still required, I'll make some tests during the next week to make a demo of it.