I guess it's lucky for Dredd then,
that evidently, Ma-Ma's device was not the kind that sent a continous signal to the explosive, but one that sent a signal only
if her heart stopped. There's nothing in the film to discount her device working that way. Nothing I can remember, at least.
But that's silly. It's missing the whole point of a deadman switch. I mean, think about it.
So either the filmmakers made a big mistake, or the character did. Either way, it's a stupid contrivance and a major conceptual flaw in the film's climax.