what would you do

I'd tell her to never do it again or she'll never see him again

it depends... a well deserved and light swat on the butt is completly different from a full-out spanking that leaves whelps.

Can you explain the situation?

It takes a village top raise a child. If you tell her to not do it again, please be sure to not do that in front of your child. Your child needs to respect her no matter what and if you chastise her in front of him, he might feel like he doesn't have to listen or be respectful, and that will start all kinds of problems with his behavior.

i think a lot of it depends on your relationship with your mother in law and what kind of spank it was and what it was for. My mil never listens to anything i have to say. So if she spanked one of my kids, i would be upset, because if she would just follow the rules that i set in general, it wouldnt get to spanking point. However, my FIL (they are divorced) is a different story. We are much closer with him, he respects my parenting, and i know if he were to ever spank one of my kids it would not be hard, it would be a light swat to get attention, and it would be for a good reason.

SHE DON'T HAVE A RIGHT TO TOUCH YOUR CHILD!! She is not the parent if she has a problem w your child she should tell you or dh and let you guys handle it. I wouldn't even allow my mom to spank my child. I will do the spanking if I feel it's neccessary but I feel it just makes the child angrier and more resentful have her do time out instead

call 911

If you don't believe in spankings and she feels your child needs one, I would let her know to never touch your child again and if she feels the need to spank him, maybe she doesn't need to watch him anymore.