What was she thinking?

[playfelt]

Kind of sounds like a passive-aggressive power trip on her part - she knew better for sure. Keep your eyes and ears out for other signs and don't be afraid to speak up and put her in her place the next time she starts something like that.

I would also be speaking to the boy even right in front of him mom to address the rules at your house and remind him that if you ever catch him so much as trying to reach the gate latch that he will be sitting in time out for the rest of the playtime and suffer additional consequences should he not learn from the first incident. It lets mom know again that her actions will cause hardship for the chid potentially and she has no one to blame but herself.

Another good option would be a padlock on the gate with you wearing the key and you can let her in and out at your schedule and is she doesn't like having to wait a bit well she brought it on herself.