I love my son and when he told me he had cheated on his girlfriend, I offered him my unconditional love and support. But I also let his girlfriend know that I absolutely did not support his choice of cheating, and that I would be entirely supportive of whatever choices she had to make to get through it. I never minced words with either of them. I never blamed her in any way, and I told him that he better let her vent, let her cry, let her ask questions, and answer everything honestly.
They lasted another year, she and I talked many times. She knew that I meant it when I told her that I loved her, she knew that I disapproved of his choices and would in NO way condone what he had done.
Sadly, she wanted to move back home (New England) where she's from and he had just gotten hired as a public defender here in the South, where he's from. They had political divisions as well, and she didn't like his smoking. All in all, I guess it wasn't meant to be. But in no way would I have turned on that girl. She was crushed.
We stayed friends for about a year and then she deleted me. I wish her well and hope that some day we get back in touch. She's a sweet girl and didn't deserve what he did to her. He still regrets it but she has moved on.