I swatted my son and daughter once each when they were toddlers, lightly and they had diapers on. They were about to do something that would hurt them. I never truly spanked them.
My mother did that to me when I was a wee one. We were walking out of the grocery store and I tried to run to the car so she pulled me back and smacked my bottom. It startled me, but it didn't hurt. She just didn't want me running out into the parking lot like that. Some older lady glared at her when she did that though.
I don't believe in spanking. We play spank in my house, as part of wrestling and roughhousing, but it's not a form of discipline I employ. I feel so strongly about it that I had it written into my divorce decree/ parenting plan with my ex. He believes in the practice, but he isn't allowed to do it or he's in violation of our legal agreement.
There are some parents who agree with it, but I think its generally the result of ignorance and lazy parenting (or parenting by an overwhelmed parent. It has been shown to cause cognitive damage, behavioral problems, and emotional problems. If your grandson's mother does it, I wonder if she'd consider learning alternate techniques. If it's presented to her a certain way, in a non-threatening/ non-judgmental way, maybe she'd be game. Or, maybe she and your son could take a class or read a book together, for the sake of having an open discussion about discipline. If she's doing it because she's overwhelmed, then maybe you helping out so much is a good thing. Being overwhelmed isn't an excuse, but when life stresses us out, we do crappy things. We're more impulsive. It's a lot harder to work through a discipline problem while it's happening than it is to swat. The long-term benefits are there, but when life goes to heck and we're stressed, it's so hard to see the light at the end of the tunnel- especially if she was raised to believe that spanking is ok or doesn't know the damage it can cause.
Benedict Arnold 
Oh HELL NO.