A friend fitted a battle system to his tank, The IR bulb fires in test mode, but not in normal mode, can anyone help

How do you know it does not fire in normal mode?
did you plug in a tamiya IR receiver (apple) on the tank before turning the tank on?
take a camera and put it in video mode and watch the IR LED wile it fires. It will light purple in the camera. Test this and come back and give results.
either something else is wrong or the MF is faulty.

Most likely is that the apple isn't connected properly. Tamiya (and other boards) lock out the emitter if the system doesn't detect the apple, in order to prevent shenanigans. If he's something other than the complete apple and harness, then he'll need to get the circuit testor out and go hunting dry joints, check his IR sensor pin outs etc.

Hes tried apple both ways round,, using a camera to see bulb as mentioned fires in test mode but not normal. , will check on harness situation

not sure that you are supposed to plug the apple in backwards Has he got another tank or a TV remote that he can zap the apple with to check its function? If he shoots a mirror in test model, does the tank take a hit? (anybody?)

Double check the plug for the apple base to the MFU, make sure none of the sockets have backed out and you have a good connection. Also could this be the SW3 switch enigma? I remember years ago when I built my JT it was acting strange and somehow the SW3 switch got moved. I can't remember what exactly wasn't working but it solved the problem. As EX Pat pointed out, putting it front of a mirror and seeing if you take a hit will help to proceed.

after alot of swapping around its come down to mfu fault thanks for all the replies

Poop

Contact Tamiya europe where ever they are. Im sure they will help you out, they stand behind their products often beyond the warrenty period as long as you bought it from a retailer in the area.
if you bought it from a forigen dealer like hong kong off ebay, you will likely have to go direct to tamiya Japan.

There is no Tamiya UK - its a just distributor, who is as much use as a chocolate tea-pot