I take it they were overreach boots? And her pastern has rubbed sore? I've used them myself in the last but I've never had that problem - at the most the boots rub their hair a bit short if you use them regularly.
Don't beat yourself up about your mare. Treat her sores and they'll heal rapidly. It just goes to show that often your instinct is better than bad advice, and that even lifelong horse people can get things wrong. Something doesn't quite fit though - did her previous owner always use boots on her? And if so, either she regularly had bloody legs, (which seems unlikely, and she would have scars to show) or something went wrong with the boots yesterday and they rubbed her badly. Or maybe he gave you a larger pair of boots than the ones he always used for her?
Congratulations on cantering with your horse! Don't let this minor complication detract from your achievement. It's great that you're feeling confident on her again.
There is nothing more peaceful than watching a horse eat.