I think you have to make sure you kill everyone in that area before you can get in to save the foreigner.
Make sure they're actually dead. Sometimes they drop and then crawl themselves into a corner, and sit there with their Pistol across their lap. Walk around the area - check in all the buildings and along walls etc.
When you get close, they'll try and raise the pistol and take a few shots at you, but it takes them a few seconds, so if you're sprinting past them they might not get a chance to shoot (and therefore give away their position). In other words take it slow, sweep and clear.