Make him pay.

You decide to give him the pink, while the easier of the choices you needed to do this in carefully so this doesn't blow up in your face.
There's the fact that Zack works in the same place and your co-workers knew that, meaning that he can't just vanish or appear at work as a woman without throwing suspicion your way.
But those were details that you'll cover as you approach them, first you needed to think of what kind of pill to order.
You could just give him a Bimbo and be done, or you could give a basic and indoctrinate her yourself or better yet use X-change's custom system to mold Zack into whatever you want.

(If you want, you can add to this branch)