It wasn't joining the roundabout, it was exiting in a similar scenario to what OP described by effectively turning right from the left lane (The potential collision point is slightly earlier as the other party is intending to go straight instead of right). The below shows it better. Blue was my father, red was the other party. The solid lines were the directions of travel. I added the dotted blue as 99.9% of people use the roundabout as a continuation of the dual carriageway.
Ah ok, I wondered if that was the route you described, was a little hard to tell.
The only thing I can assume is that the driver of the second vehicle, lets call it red joined after your dad, blue.
Obviously blue failed to to correctly approach the roundabout as he should have been in the lane red approached in. Granted he indicated but thats beside the point.
However if he was close to the next junction he would have been in the same lane as if someone had of entered from that point.
Whilst blue was badly positioned, I assume for red to have hit blue, that red entered the roundabout after blue, and as such you can see why red got the blame, they hit a vehicle already making an maneuver.
Normally the person in red is going faster, thats why they are in that lane so to hit blue they must normally hit a car already negotiating the roundabout.
Assuming blue was indicating as you say then you wonder why red didn't see that and react accordingly. Its not like someone getting in the wrong lane at a roundabout is an uncommon occurence.
Best practice for straight on is the left lane, your far less at risk of someone changing their mind and staying on the roundabout.
In busy traffic I always take the blue line, yes you may loose a little ground to a few cars, but its the safer route, as you avoid the highest risk area, (where the red and blue lines cross)
