Somewhat conflicting reports so it will be interesting to see what the real story is when the winch has been examined.
There are all sorts of failures that can occur in the mechanism including jamming and runaway out (if the braking fails) as well as electrical control issues.
The winch cable certainly shouldn't snap unless it has been seriously overstressed - although a short fall with an abrupt deceleration can shock load it. This is why the winchman should be winched upwards from the cabin before then being winched out to the casualty (rather than dropping off the doorstep or sill) and it tests the integrity of the cable and harness before committing the winchman to a vertical descent.
Winching through trees is tricky at the best of times but more so at night - however, given the nature of the injury, it seems clear this was the best course of action.