But anyway, I bet I know why it won't start and where the fuel leak is most likely.
The fuel filter is likely the problem. If they changed it themselves but didn't replace the O-rings properly it could start leaking and allow air into the system. If the vehicle sat for a while then it would replace all the fuel up top with air as it slowly leaks out.
The 1999 ALH engine doesn't have a lift pump in the fuel tank, it relies entirely on the fuel pump in the engine bay to create flow. If air is allowed in the system it will lock up the flow of fuel and the engine will crank but never start.
You'll also get that happening if the fuel tank was allowed to run empty. So all you old Beetle TDI owners, never let your fuel run out or you will need a tow as the engine will almost certainly not start until the air is evacuated.
What I would do with this vehicle to get it started would get a hand vacuum pump and small hose, hook it up to the return line from the fuel filter and pump until I see fuel and no air bubbles. That will pull the fuel through the filter, remove all air in the filter and also pull the fuel through the pump and remove any air there.
The return is the line going to the back of the car from the plastic T fitting on the fuel filter. Pull the existing hose off the T and attach the vacuum pump hose in its place. Pump only a little and let it run, don't go crazy or you'll have fuel everywhere all the sudden.