Filbert, I’d post a sign in the shared stairwell reading “Dear Burglars: Please rob top-floor flat first. She’s the one who invited you in.”
Since polite asking failed to get your point across, maybe some passive-aggressive snark will work. 
Is it expensive to get a type of knob that stays locked, and must use a key from outside to open it, while it opens from inside without a key or the need to unlock it? That type has always seemed necessary in the building’s I’ve lived in, or there’s always one knucklehead who leaves it undone all the time, and possibly propped open. An automatic closer is good, too.
Even if people are good about remembering, no one is 100%. I think you guys have been lucky until now. That woman needs to be whacked upside the head, regardless!
Today is the first time this spring I haven’t seen “snow showers” anywhere in the forecast for the next 5 days. Yay! Highs in the 60s and partly cloudy I can deal with. I promise I won’t bitch about hot sunny dry days if we get any this summer.