Did you consider asking OP if moisture was accumulating before accusing them of negligence?
I am definitely a layman, but the pressure differential would likely be pushing out if air was blowing into the room. Exception would be in high wind outdoor conditions.
If there is no obvious moisture buildup or, as I put it earlier, evidence of humidity damage, there is no problem.
If OP wants the window open and vents blocked, they can run a dehumidifier when they close their window during higher humidity periods of the night/day (to prevent any damage). You could've inquired about what the vents looked like and gave that advice in addition to helping them, if necessary, to provide solutions to block the vent or otherwise help them to keep their window open and minimize damage. OP lives in a fairly dry climate, River Side county, CA, I argue damage is unlikely to occur from having the window open.
Take the feedback. You were not sensitive to somebody seeking support.