Don’t worry, he just missed a mission queue. It does clearly say to use the computer. I missed it too and left the building after the line about the arms dealer not being there. You buy arms via the computer, and I guess the arms dealer later on gives you missions to get new types of guns. The computer doesn’t stand out, the game doesn’t give usable things a glow or anything, so its easy to overlook at first, but all you need to do is look at your mission goal to see what you are supposed to do.