Hi my friend has a kx 125 1990 bike and when you turn the petrol on it just leaks out the over flow pipe

There could be debris in the float bowl preventing the float valve from closing. If this is the case it is likely this debris may get into other passages / circuits within the carb. Or the float has become full of fuel itself and cannot exert enough pressure on the valve to stop the flow of fuel. Also the float level has to be at the proper height to meter the fuel at the proper rate of flow for the engine in all conditions. Too low and the bike will starve for fuel before the valve can respond....to high and it'll pour out as your friend's is doing.
Drop the bowl off the carb and check to see if the float is full of fuel or there is any sign of debris in the bowl. Any sign of crud in the bowl and I would suggest cleaning the carb. You may be able the clear things enough to get the fuel to stop flowing by messing about but may have trouble down the road shortly if you don't get it all **** and span.
Do some more research into cleaning the carb as there are some parts you need to be careful of exposing to chemicals like solvents and cleaners...the rubber tip on the needle valve on the float. Basic rule is don't soak any plastic or rubber bits in any nasty chemicals. Same goes for you...lol...
You can check the float level without removing the carb by shutting of the gas and draining the float bowl. Take a short piece of clear tubing (8 inches or so) and slip it over the nipple where the drain screw was. Hold the tubing alongside the carb / bowl body so the open end is up and above the level the fuel should come to. Mark a small line on the tubing say 1/4" above the join between carb bowl and body. Turn on the fuel and observe the fuel flowing into tube....once it stops carefully lower the tube until the line you made is even with the split between bowl and body. The fuel shold stop flowing just before the top of the bowl....measure between your line and the fuel level....in your case the fuel may well just keep climbing out the tube as it seems you have issues there. After you get it sorted out this info is more useful.

Air screw setting = 1 1/2 turns Float height = 15 - 17 mm Fuel level = 0 - 2 mm below edge of bowl.