OK HELP!!!

have been asked to look at a friends 1.8 tdci 115ps focus,
the battery light comes on and then the car dies with apparently no juice left in the battery, so won't re-start.

Previously (about 2 months ago) the battery (silver calcium) and alternator have been changed for new items and that was fine 'til now but has died again,

after much searching on the web have found there is a common problem with the wiring on them, i've done a continuity check on the 3 wires from the alternator plug back as far as i can go on the 3 wire loom (grey/red/blue) an it all comes back as all is ok with no breaks.
have charged the battery off the car and fitted it again and it runs for a few seconds then the light pops up, and the charge in the battery is far less than it shold be,
starting to pull my hair out now and i'm thinking it's a trip to ford to be put on the machine!

any help greatly appreciated
thanks