If it's 12" PVC just raise the inlet above the outlet
Inlet 1' above above outlet is 2258 Gallons per minute in siphon @ 6.41 Feet per second
2' above = 3284 GPM @ 9.31 FPS ( this would worry me on that outlet thing he put in if it wasn't glued and compacted extremely well) if it were me and I was in your position it seems raising the inlet would retain your siphon even if its only 1' above the outlet but if that thing he built 6' underground breaks that fitting loose under those volumes traveling @ those velocities ..........yowsers.
He should have just looked at the diagram of the siphon system and easily understood the simplicity of it and known how to properly build the outlet and I think even a second grader understand for a siphon the inlet needs to be higher elevation without introducing air
