It would make sense that for
If a is artillery shooting at Y
Y is enemy
X is friendly
That the Artillery damages x also. It would be a nice gesture for a losing player because of Arty that he forces his opponent to halt Arty fire or risk friendly fire.
However I find that since free players don't have use-of-fire orders it would be kind of awkward you would have to either move Arty back and forth or move it out of range 🤔