Meh. Cheating happens. I've never cheated, and I've been cheated on, and I can confirm it REALLY sucks, but I'm not going to get up on my little Moral High Horse and make sweeping condemnations of everyone who ever stepped out on a partner. Relationships are complicated. The circumstances that lead to infidelity are often also complicated. Romantic love is not unconditional and you do not own your partner. Some people are not hard-wired for monogamy. People go through bad spells. People go through life upheaval. It's possible to be in love with more than one person at the same time. The list goes on and on and on. It's really easy to sit on the outside of a relationship you know nothing about and make projections and judgments. Would you want someone making judgments based on a context-free snapshot of your relationship? Probably not so much.
On the flip side, reading someone's private text messages over their shoulder, there really isn't ever a justification for that. Maybe you can get half a pass if you work for the NSA and you suspect the person is plotting to overthrow the government or something, but I suspect those were not the circumstances in this case. Some guy decided to creep some woman's text messages at a sporting event, jumped to a lot of hasty conclusions, and inserted himself into a situation he had no business being in. He then hung a little bro-code hat on it and presented himself as a hero on the internet.
So...she MIGHT be an asshole. We don't know. She might be engaging in callous, thoughtless, mercenary cheating behind her kindly, trusting boyfriend's back. Or she might be texting a friend, or her brother. We really have absolutely no idea. But the guy creeping on her phone is DEFINITELY an asshole. A spying, presumptive, self-aggrandizing asshole.