If you're "friend" took a screw from your carburetor and now your scooter won't idle, he either took the idle air mixture screw, or the idle speed screw.
Here's a test to try. Let us know what happens.
Warm up your scooter and then stop it, but don't let the throttle spring back (hold it open enough to keep the engine running). SLOWLY ease off the throttle and try to get the engine to run as slow as possible without stalling. Can you get the engine to idle at any throttle position? If you can, your "friend" took your idle speed screw. If you cannot your "friend" took your idle air mixture screw.
If he took your idle speed screw, you can replace it with any other metric bolt that fits the carburetor.
If it's the idle air mixture screw, you're screwed (I couldn't resist the pun). You'll have to replace it, or get it back.
Let us know how this test goes.