Darwin: Whenever another character uses an effect that allows it choose one or more standard powers to use, after resolutions deal that character one unavoidable damage

Leech 024: Whenever an opposing character would use an effect that allows it to choose standard powers to use, you may first choose a standard power. This turn, that character can't choose that standard power that you chose

Am I safe to assume that if you activate Leech's power, Darwin is going to ping him for the one or is there loophole on him

And I love Darwin, but why does he have to screw over his own team

Leech isn't choosing a power to use though, he's choosing a power that another character can't use.

Jruffing is correct. Darwin only cares when a character is choosing a power to use from an effect. Leech is not choosing a power to use.