Similar situation happened to a male friend of mine. She told him she was on contraception (injection) but she'd stopped having it and didn't tell him.
Friend wanted to be a dad, but after he'd finished his study/qualification years.
The look on his face when he was coming to terms with such a life changing betrayal is etched in my mind.
I'm with you on that, it's horrible to see your friend come to terms with that.
The difference is that your friend knew he was taking a risk. He knew 100% he wasn't wanting kids and left a chance of it happening. The look on his face isn't so much betrayal, because she didn't hide it from him, he thought there was a 1% chance. The look is realising he's made the worst mistake of his life, as someone who didn't want kids.
If he 100% didn't want kids, he could have had a vasectomy years ago. He didn't. He could have used condoms on top of her birth control, he didn't. He wanted something he wasn't prepared to ensure. It must be a big shock for him.
As his friend, it may be an idea to tell him he needs to take responsibility for his actions, pretty quick, even if he doesn't like the outcome of them.