Scarlet letter anyone?
They weren't married.
She has sex with another guy, and he permanently* disfigures her body.
Are the two actions equivalent?
You can't call cheating on him a crime, but it's quite possible that what he did was a crime, so in the eyes of the law they aren't equivalent.
People would probably be on her side if he cut her skin with a knife and left scarring. The only thing that seems to make this 'OK' for some people is that she wanted a tattoo - even though it isn't the one she got. Rather than a few knife cuts, she gets a billion needle stabs in a scarring/inking pattern she didn't fully endorse.
I could see him getting away with putting an easter egg into the tattoo she wanted, or changing a small tattoo in an out-of-the-way place completely, but this is something that she won't easily be able to hide, or cheaply fix.
So without hearing more details about the situation, I'd say he went too far.
*even the best tattoo removal techniques today may leave some shadows and/or scarring, and some affect natural pigment, making it harder to tan/easier to burn, etc, etc, etc