He'll chop and slice you, cut and dice you, eat you up whole...
From the rulebook :
Rulebook p. 22 wrote:
When played - A type of ability on non-sorcery cards. A "when played" ability happens immediately after the card containing it is played and enters a battlefield.
So effectively, the Widow is still at full health when you resolve her "when played ability", and you can only weaken her afterwards.