60Hz disrupts the heart and 50Hz doesn't? I have a hard time believing that slightly lower frequency makes such a large difference.
I have heard that AC, 50 or 60Hz, is more likely to cause fibrillation and therefore death than DC, because DC is more likely to just cause the heart to lock up until after the shock ends. Subject to damage from heat, of course.
Also, my mantra has been Volts Hurts, Amps Kill. Voltage alone can still kill you, but it requires being at extremely high voltages (At least 20kV, though this is a guess, I know that I've watched a coworker get shocked by about 15kV without lasting harm, though it hurt) and usually results in a major explosion where the concussion kills you. However, that is assuming no current.
This is a dangerous thing to say on any forum with newbies. As has been pointed out, 12V can shock you. If you were to pierce the skin in two places, say both hands or a hand and opposite foot, 12V could electrocute you (electrocute = kill, shock is if you survive).
Drop a wrench across the 240Vac line coming into your house, you'll see an arc flash with a concussion that can kill you and set nearby flammables ablaze. No way, shape, or form does it require "at least 20kV".
Your coworker was hit with, let me guess, a neon sign transformer? Those are current limited. As low as 10mA. FYI, there is no such thing as voltage with no current.
Look up arc flash on Youtube. In one case, there is a story of a worker who thought he was measuring 440Vac and put the wrong meter across a 2kV line. The resulting arc flash jumped from the meter to him and then across the lines, set his clothes on fire heat to toe, blew him back into the wall, and eventually killed him a day or two later.
Once an arc forms, the resistance is relatively low. Your flesh, once you get beyond the skin, has only a hundred ohms or so of resistance, even from one arm to another. I did some testing with a current limited high voltage power supply that I designed and built. And that is at very low currents. At higher currents, as someone pointed out, cell walls rupture and the resistance goes down.
And even with relatively casual contact, 60V can do you in. Get a hand locked in place by the current, and you get to hang on the wire and slowly cook to death. Or muscular reaction throws you into something, causing secondary injuries that may be fatal.