I've found a small screwdriver wedged between the teeth of the cogs can be used to carefully pry them back counterclockwise one at a time.
You'll also want a metal file to smooth out the splines on the freewheel which will be a little messed up where the cogs dug in.
This is a very common problem with light alloy cassette carriers, I've always been able to salvage things by working carefully.