What would make a difference is if there were lanes marked and if the markings indicated left-turn or straight on, right or whatever.

Exactly.

In general, assuming two lanes on approach and two lanes on the roundabout:

1) To go left stay in the left hand lane.

2) To go straight ahead use either lane.

3) To turn right use the right hand lane.

But the above can be overridden by lane markings: I can think of several roundabouts where you can use either lane when turning right, essentially because the roundabout is a kink in a main road with additional exits for minor roads.

From the sounds of it, your wife was in the wrong (left hand) lane for taking a 4th exit, but without seeing the layout of the road, the marking on the road, and any signs before said roundabout showing lane demarcation for which exit is to be used, it's not cut and dried by any means.

There are always relative speeds and sensible driving to be taken into consideration in such cases, was it reasonable as a stranger might to be proceeding reasonably carefully around the roundabout and making for the exit, when along comes billy whizz taking the racing line at 50mph at a busy time of day as one might at 3am with not a soul to be seen.

This case calls for witnesses and carefully going over the road layout markings and signage, a front facing camera in your car might not actually be much use in this case other than confirming if what your wife did herself was reasonable and that an alert driver should have realised and had ample opportunity to avoid hitting her.

Just because someone has got themselves in the wrong lane doesn't mean they are automatically in the wrong, ideally an independent witness with a clue, or at least two angle video recording (front and rear in my car for instance, our lorries have front and side facing cameras) would be handy here.

Edited by gordonbennet on 05/03/2019 at 10:19