Well done on her for getting out and navigating the changing table. I really hope that she doesn’t let some other persons terrible mood affect her, as that really was not about your OH.
As others have said, the lock is there to be used, and take it on a case by case basis - if you feel the need to lock, then do so.
I remember the first trip out with my baby and having to use the changing room and yes, I did lock it as I felt I needed to in order to calmly change my baby without interruptions / distractions / possible judgement or a stranger in the mix. I also had my husband in with me - it was our first time working through the scenario.
I will re-iterate - well done to your OH for taking your baby out and getting through it and navigating an out of hand woman’s terrible people skills and lack of kindness.