There might be a serious risk to relevant persons in the case of a fire in her flat with the front door left open. However, not many people do anything other than keep their front doors shut all the time so the resident would have a good defence by confirming that, in fact, she always keeps her front door shut. Then, the only serious risk to relevant persons involves her having a fire and leaving her door open. Well, that is possible, anything is possible, but you have to think about the arguments in court over whether this is reasonably foreseeable. And if it is not reasonably foreseeable then there has been no serious risk to relevant persons over and above the risk that would exist if a self-closing device were fitted to her front door.
As for the electrician, he could have a good defence in stating that he was simply following instructions and that he has not been given sufficient information about the fire strategy in the building to enable him to determine whether or not the works he had been assigned comply with that fire strategy. It is easy for him to sidestep responsibility.