When a friendly criminal shoots you a few times, they get yellow-carded temporarily. You can take out friendly criminals without becoming a traitor if they're yellow carded. When they're yellow-carded, you have a limited amount of time to shoot them.
The Subway is a good location to do this, as described by Destrier. As you're going for the evacuation point after looting, your team will take cover on the left-hand side. Stand between them and the cops coming down the stairs so one of them gets yellow carded. Then, let yourself be downed and kill the yellow carded team member. The dog that comes to attack you can quickly do this. Also, once you go up the stairs there will be several enemies shooting at you. This also provides a good oppotunity to stand in the line of fire of your friendlies and also to be downed quickly.
Another good place to get this is the Harbor map. Eventually you'll come to an area with a car right in the middle to take cover behind. Quite a few enemies will begin to shoot you. Your friendlies will be behind the car shooting at them. This provides you with another good opportunity to stand in the line of fire of your friendlies and allow one of them to become yellow carded.
Search for locations where your friendlies shoot consistently and for a long time at enemies in a specific location. Also search for locations where there's a good balance of cover, so you aren't killed, and enemies who can down you quickly, giving you more time to shoot the yellow carded criminal. This takes some patience, timing, luck and practice. You should be able to obtain this achievement in a few tries.