I'm playing with my friend and we have an argument about the mention of "At the start of his activation" on an upgrade card. He is playing Kanga, and he wants to declare an attack against a monster, take a wound by using her ability (May take a Wound instead of exhausting a card.) to not exhaust the card, and then use the effect of Cloven Hoof (At the start of his activation, this Hero may take 1 Damnation to heal 1 Wound) before finishing his attack.

He claims that since his attack has not been performed yet, and he did not perform any movements, he is still at the "start" of the activation.

I'm claiming that he cannot do that since declaring an attack is in the "attack phase" and it is no longer at the start of the activation.

Can someone tell us which one is the correct case for this scenario please? We can't seem to find anything in the rule book.