I don't see anything in your post that makes me believe that if Karen Sandler had access to the code she could make improvements to the device for her particular situation.
First, as another poster has noted, modern implantable devices are extensively configurable, and yet most of them go in with the default settings, because the cardiologist/surgeon don't know enough about each device to tweak the settings. So it is quite conceivable that it could be already be configured to deal properly with a pregnant woman's racing heartbeat.
Second, all of these devices walk a hazard/benefit tightrope. You are dealing with devices that can kill the patient if they fail. The patient might die due to the ordinary surgical complication risk that is always present. The device might function but not actually help them because of their particular physiology. So the validation of the device talks a lot about risk and reward, and the testing will focus on the population most likely to benefit. It is likely that pregnant women form a miniscule market for this device, so they may be considered an off label use - something that was not studied and about which nothing is known.
Think of pharmaceutical ads, and how often you hear the phrase "women who are pregnant or thinking of becoming pregnant should consult their doctor". That tells you right there that either pregnant women weren't studied, or that they have additional risk factors because of the pregnancy.
To think that access to the sourcecode by an interested layperson could make the software meaningfully better is a stretch. Perhaps getting access to the programming manual for the device would help, but that doesn't require access to the source code.