Well if the kid has been flashing lights, interior lights on all day, radio going, the battery is probably dead. If it's an old battery, they sometimes don't recover. Jumping a diesel is often an issue with cheap jumpleads, and the car just won't start.
Try putting the jump leads on to the Laguna, and leave the other car running for 15 mins on a fast idle, it should charge the battery in the Laguna enough to start itself. Basically you're wanting to give the battery a bit of charge so your "good" car's battery and alternator isn't trying to start and charge the Laguna, possibly though insufficiant jumpleads.
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