An allay which has been given an item cannot normally be injured by the player who gave it the item; however, a sword with Sweeping Edge and Fire Aspect sets the allay on fire if it is caught in the line of the sweep attack.

In my case, I used a Sweeping Edge III sword with Fire Aspect. (I tested afterwards, and I could not hit the allay directly with the sword.) This occurred repeatedly when fighting groups of zombified piglins, blazes and wither skeletons. I am unsure if this applies to swords without Fire Aspect.