Undertaking is not a specific offence and never was. So something else had to make it 'dangerous driving' for the charge to stick. It could be that if your friend was in lane two, and lane one was vacant the whole time, then he moved to lane one in order to 'undertake'. In that circumstance it could be construed as dangerous driving because he cannot use the defence he was driving in the correct lane (lane one) and the car in lane two was driving inappropriately slowly in lane two.

It would be good for a case like this to be appealed to the Supreme Court so that passing on the nearside could be clearly legalised or not. It's hopeless relying on decisions by stroppy coppers and useless magistrates.