Basically you wait by the van with Matt in it for around 30 minutes. This does no include the time it takes you to get from the start to the van, that's counted seperate. Timer starts once you get past the fence to the van area.

After 30 minutes he should get out and just complete the heist :D

Good Luck, takes a lot of time and patience.

Extra Facts:
You can place the gas down and get the achievement, just dont shoot it.
It takes 30 minutes to complete this on every difficulty