I'd go for swopping them over. If that side is then down I'd take the springs back and get a pair that are equelly matched, they may look the same physically but they may havr been treat differently. I'm amazed the law hasn't pulled him up if he's driving it.

Im sorry but I don't believe people drive around like that and are happy with it. If i remember correctly, on the Leons, the drivers side sits ever-so slightly higher to counter the weight of the driver, dont know if its the same for fabias. But thats not normal, it looks like 2 inches,

Mines the same!!! The spare wheel sits on the low side

A 15kg spare wheel positioned slightly over one-side is not going to make a difference

If the rear springs are a matched pair, then as I have mentioned above, I still reckon you need to look for the problem on the front suspension which is tipping the car to one side.

DB.

this happened on my mk4 golf 20vt when i put the coilys on,real axle bushes were shot ,more so on the side what was the lowest,replaced them with polys and it sat fine

I had a similar problem on my old corsa b on coilies. The rear bush was shot on one side ( the lower side )

What's the camber angle like? Could be a bent rear beam or stub axle.