If 2 people are in a house and person A opens a spare room window when they get up and then goes to work then person B goes to work an hour later leaving the window still open who is to blame?
A's argument is that the last person to leave a house should check all doors and windows
B's argument is that they were not aware the window was open
Bearing in mind that if they are burgled without entry being forced ie, through an open window, their insurance policy may be void. It's not quite a trivial matter.
If the window is normally closed then B wouldn't expect it to be open so probably wouldn't bother to check. If A opened the window, knowing that it was normally closed, surely they should have either closed it themselves or mentioned it to B.
It depends on the agreed house rules. I have more sympathy for B. A benefit of having one's own dwelling is one doesn't go opening windows unnecessary. A should've informed B that they'd left windows unexpectedly open. Meanwhile blame games only cause ill feeling. Learn from the experience and agree a better arrangement for the future.