It’s perfectly fine to have windows open with indoor cats, they are curious but they won’t jump out because they are generally quite scared of what they see out there. Plus you can get locks for windows, like child locks (I know her windows are old) so they can’t push them open too far.
With litter tray in the house, you’d want a window open from time to time. It’s just constant excuses and reasons to moan. She could go and buy some Dyson fans which are an investment but she can use for heating and cooling then move around - there are solutions but she loves to moan. She could probably just say they’re a business cost as she “works” from home. Job done