Freelancer 2 alternator belt and starting issue

Hi looking for a bit of advice ref my son‘s 2013 Freelander 2 td4, on his way to work his aux belt went he had the AA out and they replaced it and when they started it the car started for a few seconds then cut out and the belt had started to shred the belt again so the AA arranged recover to our home address, the AA guy said it needs a new alternator, which we are going to get fitted but the issue is why the car will turn over but won’t start has anyone any ideas why it won’t now start, we have tried jump packs to boost the battery.

Has it damaged the crank sensor?

I'm guessing the lack of valid output from the alternator is preventing it starting. That's just a guess though. It's also possible the thrown belt has damaged the crank position sensor wiring, which would prevent it starting.

Also booster packs shouldn't be used on modern vehicles, as they have battery condition monitors, which get confused or damaged by sudden changes in battery voltage. If the battery is more than 4 years, then it's past its best anyway, and will likely fail suddenly leaving the vehicle stranded.