My fathers airsoft gun is always firing full auto?

Sounds like a couple of things.

You have two problems. One being that it shoots full auto when semi is selected, which is easily solved, and the other is it shoots without the trigger being pulled.

The first one is the cut off lever. This is what stops the motor and disconnects the circuit after one complete cycle, meaning only one shot should be fired. This wears down over time (especially if you use a high FPS set up on semi auto), so eventually stops cutting the circuit and allows the gun to fire in full auto when it shouldn't do. They are cheap and easy to replace, get a good quality one (like Systema) and it'll last a lot longer, I can't remember exactly where in the gearbox it is and I don't know what gun you have so it'll be different in most gearboxes.

What causes the gun to fire constantly until the battery is disconnected is bad trigger contacts.

I think the plastic around them may have melted together, meaning that they are constantly contacted so the circuit is always complete (unless you remove the battery, or break it by switching to safe). This means you'll need to completely replace the trigger mechanism, including the contacts. Which is a bit more difficult as it means either new wires too, or soldering the old wires onto the new trigger mechanism.

I can't think of anything else it could be.

If you can't solve the problem when you open the gearbox later, take some pictures upload and link them, then I'll add details to see if I can help some more.

Airsoft Guns Full Auto

Sounds like something in the gearbox got jammed probably something to do with the trigger. I have heard of this happening with JG guns. It just jams then they move the fire selector switch around and it stops firing full auto all the time.

might be a bad selector switch, you should think about adjusted the selector switch. here is a video http://www.airsplat.com/airsoft-gun-troubleshootin... that might assist you. should be the fourth one. i would not subject disassembling the gearbox though, it will be very difficult to put back together unless you have some real experience doing it. you might want to send it back for a repair or something.

sounds like the trigger may be stuck, or it's somehow making a complete circuit. If it's an ARES, then that would be the clicker got stuck down, but on most AEGs it would be the circut switch

It's possible yes

check the fuse and trigger for any jams

Yes, it might be right

I think the more important question is why a grown man is playing with toys :/