Whilst I cannot find any particular evidence relating to British soldiers smelling saddle sores from a distance or the French cavalry having saddle sores specifically there is plenty of evidence to suggest that this may have been the case.

I'm not an expert in horses, so I don't know how badly saddle sores smell or whether or not that was simply artistic license on the part of Bernard Cornwell. However, the French army during the peninsular war went through a huge number of horses due to the nature of the war and due to the treatment of the Horses.

Now it will vary from nation the nation. The British had a problem with their soldiers selling part of their horses food ration to buy alcohol. Resulting in very poorly horses. In contrast the Germans were renowned for their treatment of their steeds. Records show that when they had inadequate fodder they gave their bread rations to their horses.

Did the French likely treat their horses worse than other nations? As a general answer, probably no. However, in regards to the Peninsular war where the French were fighting a brutal guerilla war which cost them huge numbers of men and horses left the cavalry stationed their with insufficient or inadequate replacement mounts. As a result these horses would frequently be in a pitiable condition including saddle sores.

This would apply to any unit which has been over-marched, under-supplied or improperly trained in caring for their horses. So the French stationed in the Peninsular suffered especially. Perhaps not due to lack of training but due to their inability to properly care for their horses in the circumstances.

Napoleonic Light Cavalry Tactics - Philip J Haythornthwaite

Napoleonic Heavy Cavalry Tactics - Philip J Haythornthwaite

British Cavalryman 1792-1815 - Philip J Haythornthwaite