Houston here (seriously :-))
What type of bad driving is it? – behavioural (boy racer, not paying attention etc.) or ability (genuinely poor driver but does pay attention to the road).
If the former then honestly talking to them about the way they drive would be the best thing and hopefully make them think.
If they are genuinely a bad driver despite focussing on the task then the best option would be professional training.
If somebody criticised my driving I would like to think I would take it seriously. I think I am a decent driver (i.e. safe without being so slow I frustrate others) when I pay attention but I have had a few lucky escapes when I was not fully focussed on the road and I will freely admit that.
One non-confrontational approach could be to say you are thinking of doing an IAM course and ask the person if they want to do it with you.
http://www.iam.org.uk/