It's not a problem with the light fitting, it's a problem with your friend.
You've got a loop in lighting circuit. Very simple and I'll try to explain it in laymans terms. Circuit starts at the CU and daisy chains from one light fitting to the next until the last fitting on the circuit. For the basic circuit you have two cables to each fitting, one in from the previous and one out to the next, until you get to the last fitting which only has one cable in (obviously).
Each of these cables should have three conductors, a live, neutral and an earth. Quite simply, the live cables join together (WITHOUT the live of the light fitting), the neutrals join together (WITH the neutral of the light fitting) and the earths join together.
Now, this is where it goes t!ts up for most people. You then have ANOTHER cable connecting the light fitting to the switch, again twin and earth. However, the conductors of this cable SHOULD be live, live and earth. Basically, two red (or brown) conductors and one bare, ready for sleeving. Aternatively, and more commonly, it's live, neutral and bare and the neutral (black or blue) is sleeved or taped as live (red or brown). The same should be done at the switch. HOWEVER, even more commonly the neutral is not sleeved or taped and therefore is not identified as live.
One conductor of this third cable (or second in the case of the last fitting on the circuit) should be joined to the live cables on the circuit (the permanent live) and the other (which is the switched live)should be joined to the live conductor of the fitting. The conductor connected to the live cables then carries a live feed to the switch, which when the switch is off, terminates there. When the switch is on, the live feed is connected to the other conductor of the third cable and power is carried to the live conductor of the light fitting. Voila. We have light.
What your friend has done is connected the live conductor of the fitting to the permanent live of th circuit, hence permanent light.
Now you also have an additional cable to contend with. This will probably be a simple spur off the middle of the circuit to an additional light fitting elsewhere. Basically you'll have two fittings on this circuit that only have two cables instead of three. This additional cable simply connects to the other lives and neutrals of the circuit cables.
You will now need to identify which cable is the switch cable and cable up according to the above which I hope is understandable. If you have any doubts, please, call a spark. Yes, it's simple, but if you know not what you're doing then you could do some damage. For christs sake, don't get your friend to touch it again. He obviously thought that he knew what he was doing because he can wire a plug. Obviously he didn't and didn't even have the nouse to take note of the existing wiring configuration before disconnecting from the old fitting.
To identify which cable is the switch cable you need to switch off the power and check for continuity between the conductors in the switch and the conductors in the fitting.
Good luck.
