It's the responsibility of the driver in the outside lane (doing the overtaking) to only move back in when he/she is sure it is safe to do so. In this situation, it wasn't safe because there was a vehicle in the inside lane wanting to move out, which the first driver would have realised if he/she had been in possession of any brain cells, even if the vehicle in the inside lane wasn't signalling. So, the driver moving from the outside lane was in the wrong.
However, when you are moving from the inside to middle lane on the motorway, it's always best to assume that any driver coming up behind isn't in possession of any brain cells, and proceed with appropriate caution.
Indicators don't have anything to do with priority. If you're in the inside lane of a motorway and you put your right indicator on, you're just saying to other drivers, "I'd like to move to the middle lane." They can either respond to your signal by giving you room (if they can) or ignore your signal, but neither is wrong.