Richter can be found in the final stage of Chapter 13, Assault, on a counter in the room furthest to the north-east, lying next to a non-usable silenced uzi.
The name Richter is German and means "judge." It's also the name of Michael Ironside's analogous role as the secondary antagonist of Total Recall, where he's a low level enforcer with little knowledge or power, tasked to kill an amnesiac man who he's secretly affiliated with.
In Act 5, Richter is shown to already have the mask by April 2nd, 1989. His acquisition of it is never shown. The mask provides no bonus, either because his Uzi assassination was a one-time use or because he hadn't began using one by April 25th. It's also possible he's lying to Evan and exaggerating his difficulties for sympathy and plane ticket money.