I’d say the fact that your director has specifically mentioned that he wants to know what your colleagues contribution has/hasn’t been means he’s already ‘onto’ what’s going on.
Just be honest – say you have been ‘covering’ for your colleague, and say something to the effect of “well, if I hadn’t picked up the slack it wouldn’t have got done” and “I think it’s OK to help people up to a point, but not when they just won’t/can’t learn and it happens all the time”.
Knowing how some directors’ minds work – this might be your only chance. He’ll probably be viewing this as a one-off judgement to clear his conscience and the air in general. In my experience, most directors don’t want to leave any meeting/process with unanswered questions. The fact that your director is taking an interest makes him pretty good in my opinion – a lot don’t give a monkey’s how much strain people are under, just so long as the work gets done.
Also, ask yourself if your colleague would lie and play up their contribution if they were giving the report – it sounds like they’d stiff you given half a chance – no reason to be sentimental.
Don’t sweat it – the chances are your colleague may not be cut out for the work anyway – if they can’t cope, it is in their long-term interests to get it sorted as well. What would happen if you left?
Few big assumptions there!