Getting a lift off a bad driver

[donkmeister]

I am currently in a friend's car on a long trip. The standard of driving is... not good. But, good manners and a wish not to be known as a wannabe-driving instructor have prevented me saying much apart from "I think your lane merges here", "I'm not sure you should stop on the motorway, perhaps there is a gap further down for you" and odd yelps.
So, do you point out driving faults to someone giving you a lift?