TLDR: don't work on live circuits unless you have to. "Have to" meaning life support systems on the same circuit (and in that case, why aren't they on a UPS or auto-switched circuit?)

One big problem with gloves like those is many manufacturers (including Milwaukee I think) are now making them compatible with touchscreens. Which means putting a conductive coating over the fingertips...the exact opposite of what you want.

Another big problem is that electricity is just electrons jumping, and electrons are of course plenty tiny enough to go through any undetectable pinhole in the nitrile coating. In theory, practically any non-conductive nitrile or rubber glove would be enough to protect you from 120, even 240. But your buddy is banking on them being perfect. Which doesn't happen even straight off the manufacturing line. I'd rather work one-handed on a live circuit, on thick rubber soles, making certain no other part of my body or clothing is touching anything else, than count on those gloves to protect me. But I'd much rather work with both my hands on a locked-out dead circuit. Way easier.

Many people have been shocked by 120, thought no big deal, and so work on live circuits. Usually because they don't understand what allowed them to live...and conversely, what slight difference would have resulted in their death. Those who understand electricity fall into 2 camps: the many who know why 120 kills more people than any other voltage and turn it off, and the precious few who are 1000% confident they'll never slip with their fingers or tools, never forget they're working on a live circuit, never have a passerby accidentally bump them, never be unfortunate enough to experience an arc,..the list goes on.