I'm pretty sure the massive lag on take-off is just a feature of the multitronic box... I remember with the 2005 A4 multi I got rid of a couple of years ago, it was a case of stomp-wait-wait-wait-go (which was a real bar-steward when trying to nip into a across traffic, enter a busy roundabout, etc).
If it is "clunking" when first selecting drive/reverse and moving off, or just plain refusing to select a gear sometimes (is that what you mean by "floor the throttle and nothing happens"?), that would be more of a concern... in that case, it can sometimes be the control unit (which cacked out on my A4, and cost about 900 to replace at an independent garage).
If it was on its original 2006 box, I'd probably say try to trade it for a manual or a tiptronic (quattro), as the earlier multitronics are notoriously fragile, but seeing as it has had a replacement box, presumably of newer vintage, your mate will probably get some life out of it (as long as he doesn't thrash it too badly, and changes the fluids like clockwork). Assuming it doesn't already have issues, of course.
Hope something in that load of drivel is helpful
