Not to be a spoil-sport, but I'd rather not read about something happening to either one of you because someone didn't say something.
The practice of using a ground lift on an amp is bad enough, but clipping the wide blade (called Neutral) on the ground lift adapter and reversing it makes it far worse. Neutral is grounded through the power panel, the other (called Hot) is not. By reversing these, Neutral is actually Hot inside the amp and is no longer protected by the fuse and switch as the amp was designed. This is what made the two prong plugs in old amps such killers - Neutral and Hot could be reversed. In this case, if the reversed Neutral shorts inside the amp, 120v gets connected to the chassis, and there is no fuse to blow. RIP.
Rubber soled shoes will not prevent electrocution if there is a voltage difference between the two pieces of gear that you happen to grab a hold of. You simply complete the open circuit - grounded or not. Current will flow through you in whichever direction has the lower voltage reference. Different gear has different ground references - they are not always 0 volts. With no ground (it was lifted) to "sync" them to a common reference, one "gnd." could be at 60 volts and another at 10 volts. The seemingly small 50 volt difference will kill you if the current is high enough.
Joe has more practical experience with this in stage settings than I ever will, but I've been working on gear for close to 30 years and have been zapped more than my share (my wife says I never learn). Always use a ground or an approved isolation transformer. I wish the amp makers would take a tip from the medical field and use isolated power. All medical gear is required to use it for this very reason.
Guitars: '79 LP Custom, '01 Dickey Betts Goldie - 80 of 114, '00 Chandler Lectraslide
Amps: '00 Marshall 1987x, '70 Marshall 1959 SL, 4x12 JBL D120s
'64 Vibroverb - JBL D130, '66 Super Reverb - CTS Alnicos
'77 Peavey Deuce (great for melting stubborn ear wax)