After reaching Ngumba Industrial Zone, head to the main building and choose the door near which you can see some blood stains. Walk through the rooms. You will reach a place where Shabani is lying on a bed.
When the cut-scene ends and Man on Fire appears, start running towards the exit. Don't stop not to get hurt by the fire. When you're outside, you have to decide how you want to act - you can try to stop the attacks of Man on Fire (you can't defeat him completely), which will allow you to complete an optional objective, or you can endure the confrontation waiting for an opportunity to evacuate.