<human>: For some context, I've known the guy (call him Brian) and his girlfriend (call her Katie) since elementary school. We were part of the same friend group back then, but were never close friends. I've stayed acquaintances with them through the years, however. I never actually hang out with either of them though. I don't even have either of their numbers.

4 days ago, a close friend of mine told me that he heard a rumor that Katie hooked up with and had sex with a guy at another school in our town. The source of this rumor was one of Katie's friends. Nothing too substantial, and no real evidence, so I decided to ignore it. Besides, it was none of my business anyways.

Yesterday, I was hanging out with some friends near a trail, and I see Katie and the guy from the other school walking hand in hand, very close together. That pretty much confirmed the rumor that I heard, and I texted the friend that told me about this.

Katie and Brian have been dating for almost 3 years now, and I'm feeling really bad about this. On one hand, I think he deserves to know if his girlfriend is fucking another guy. On the other hand, I don't know either of them too well and high schoolers are really gossipy and I don't want to get super involved in this.

I have friends that are closer friends with Brian, and I've thought about involving them, but I'm not really sure that involving other people is the best way to do this.

What should I do in this situation?

Can you condense the text into three sentences or less?
<bot>: I found out that the girlfriend of an acquaintance of mine was cheating on him. I'm not close friends with either of them, but I think that he deserves to know. How should I proceed?