I have two things to say on this
Masturbating in the workplace is a serious offence
Even if it is in your lunch break in a cubicle with the door locked?
What you need to do, is wait until he is at the sink, run up beside him and piss in the sink next to him. The curve of the sink will cause the piss to naturally curl, leap out of the edge and soak him right on his crotch.