Registration and Posting Agreement
... confirm the battery connections are clean and tight, then make sure the ground cable where it attaches to the engine is clean and tight. Physically unbolt them and confirm it's not arcing inside any of the connections. I would also connect a multimeter to the battery and watch it as somebody else tries to crank the engine. It's not uncommon to have a battery that checks ok with a meter at 12.5 volts and to drop to nothing when placed under load.