When does Mark of Subversion fire?

There's an ambiguity in this spell's definition...
Is that 'ally' an ally of the enemy caster or an ally of yours? That is, would Mark of Subversion fire if:

a) you cast on an enemy Monk and the enemy monk heals another enemy, or
b) you cast on an enemy elementalist and the elementalist fireballs an ally of yours.

The spell is triggered when the target casts a spell on themself or one of their teammates. Basically their ally (including themself), not your ally.

So in your examples, "A" would trigger it, "B" would not.

It's basically an anti-enchantment/anti-heal/anti-hex removal spell.

Thanks