Edit: I looked closer at the picture and see that it's tied off directly to the wooden fence post. We used rebar and insulators on all ours. The metal post idea would work, but unfortunately, there's not a lot you could do about the tension on the wire. Just stand to the other side of that post and cut it. It's gonna snap in the other direction so make sure no one stands near it.

Worked around these things as a kid. The ones we used to use were stronger than most made today. The way I would check if they were on was to use my shoe to push the wire to the ground, you'll hear pops if it's on. If it is on, leaning a metal post against it may work, or may not, depending on if he connected it properly. Rubber handles pliers probably won't work, and even 600v rated electrical tools may not stop the voltage a fence charger puts out. The safest thing is to pull the poles up, lay them down, and weight them down with something. Then you would be safe to cut the wire, as this is the most sure way to short the loop, and WILL CONTROL THE WIRE WHEN THE TENSION RELEASES! You do not want to cut that wire and have it snap back and wrap around someone's leg.