Well, traditional latches for a sash window usually just turned round to stop the window sliding up, HB! Not sure how you could secure it from outside, but who knows? i've never tried!
Remember, they were old windows Snow, they had gaps in them, if he admitted using a knife to get in, surely he could use a wire or string. I’ve not seen such, but Caroline said she’d seen a video where it was possible to lock the catch from the outside using string?
What would you advise your parents do if their property was so easily to break into? The guys been through every Window to get in, if it was me I would tell them to do something with those windows, I would say, “look there’s guns lying about in the house, if I can get in, so can someone else, and who Knows one of these days someone might climb through the window and rob you, but worst of all use the rifle to shoot you in your sleep”……….Or maybe he didn’t want to warn them for some reason?