As a Captain, this is - unlike many other cases - a very black and white case.
Over mainland Europe with someone slumping away, family on board who can tell about her general health and inform of her allergi, a diversion is called for.
Unless it is severe weather or Low Vis all over, or the area infested with hurricanes, it is possible to be on the ground within 20 min anywhere overhead Europe's mainland.
This is absolutely tragic on all accounts, and a diversion might not have changed anything, but not trying while flying where they were, is poor decision making.
Safety first.