Maintenance Inspector reported a fuel line was not properly reconnected after repairs; resulting in a engine fire on startup.

Narrative

Repairs to a leaking right nacelle fuel tank were being completed by a 20 year experienced apprentice/tech. Defueling of the tank was required and was completed twice during the repairs. The first time via disconnecting the right engine fuel line and using the boost pump to offload the fuel from the tank into the opposite wings tank. Right nacelle tank was refueled with fuel from the left tank via a fuel hose from the left engines fuel line with the boost pump. Fuel line for right engine may have been removed/disconnected to facilitate defueling the tank the second time and possibly not fully reconnected.All panels were closed and leak check of area in work revealed no leaks. Left engine fuel line was checked to verify it was reconnected. Right fuel line was not inspected as the supervising tech (reporting party) was not fully aware the right line had been disconnected the second time.Aircraft was returned to its hangar. Owner conducted a preflight (according to his statement) in his hangar the night before the proposed flight; checked engine oil and FCU (Fuel Control Unit) linkages and did not note anything wrong in the right engine bay. Aircraft was on the FBO ramp and the right engine was being started. Pilot advanced condition lever to low idle at the appropriate engine NG and immediately after a explosion/boom was heard and he moved the lever to cutoff and began evacuating the aircraft and closed the electrical gang bar in the process. A flash fire puffed out of the exhaust stacks and below the nacelle and a fire was seen on the ground below the nacelle. The engine spooled down as the starter cut off and the fire was extinguished by the FBO line personal as the passengers were evacuated. Inspection procedures existed (second set of eye to inspect all critical work) and were followed except for the right fuel line not being reinspected due to it being taken off the second time and not being clearly communicated. Additional procedures are being implemented to include a removal log/checklist of any critical item (fuel/oil hoses; flight controls ETC) that can be checked against to verify installation and/or inspection.