I'm an 8 wheel tipper driver too, so I'll try and be objective.
I think, but I'm not sure, that it is still an offence to show a wrong signal. Having said that, as so many people don't use any signals nowadays, I make sure that people are going where they indicate they are going! How far up the road was the entrance? This may have a bearing on if it was reasonable for the HGV to assume your wife was pulling in or if he should allowed for it being the end of the signal off the roundabout. I know some Little Chefs are right on the roundabout, so obviously extra caution should be applied. Sorry, this isn't very helpful is it. I wish I could remember the legalities of indicators. Probably going to be a thrash out by the insurance companies.