MIL cut my sons hair

I need to get this off my chest because it’s still be bothering me! Sometime last month my son (23 months) spent the night with his grandma like he does sometimes. He comes home the next day and I notice something off, HIS HAIR WAS SHORTER!! So my fiance calls his mom and asks if she cut his hair and she said she did it because she thought my fiance wouldn’t have enough time to do it since he was so busy with work. I WAS PISSED! pissed wasn’t even the right word. Long story short, I brought it up to her. We don’t have a bad relationship, she does a lot for me and my kids. BUT she does do a lot of stuff that pisses me off. when i brought it up to her I told her a boundary was crossed and that she should’ve asked us before she did it. next thing i know she’s trying to flip it on me!! and says “well i feel like im being taken advantage of” and she just keeps going on when i told her all she needed to do was ASK. and get this, she said she would’ve understood IF we never cut his hair before. but because we cut his hair, she didn’t think it was anything wrong. DO NOT MAKE DECISIONS FOR MY MF KIDS!!! they are MY kids. not yours. the audacity of her to get upset at me for something SHE did wrong