I am pretty certain the code to "unconstrict" all constricted victems of a constrictor should go in Actor.lua -> function _M:die(src)
But the problem is, in death there is a src, but not sure if there is a constriction victems... Ie. If the constrictor is constricting my summoned unit, and I kill it, I think it's easy to remove constrictions from myself, but removing it from the constrictee... might prove difficult.
You'll have to remember the target when you are constricting the guy (in timed_effects.lua)... Hmmm... tricksy...

I wanted to supply a patch, but I'm not comfortable with the code (engine) yet.

But I'll learn as I go along.
EDIT: Oops ... this is the wrong thread!