Should I tell someone I don’t know, that their partner is repeatedly cheating on them?

I need some outside opinions because I’m really torn about this.

A couple of years ago, I met two DJs at a festival. One of them made it pretty obvious that he was interested in me. Nothing happened between us, but we stayed in touch.

The following year, he put me on his guest list for the same festival. It was pretty clear that he was hoping something would happen between us. It didn’t, mostly because my friend was glued to my side the entire weekend and we barely had a chance to be alone.

Later that festival, I saw him making out with another girl. I wasn’t interested in him romantically at that point anyway, but what really bothered me was that I knew he had a girlfriend… who is also the mother of his child.
After that, he completely ignored me. Then, whenever he saw me around the festival, he’d suddenly panic, and afterwards he started blowing up my phone with messages. I ignored him.

Eventually I told him that what I found disgusting wasn’t that he hooked up with someone else, but that he was cheating on his girlfriend. His response was basically, “You misunderstood the situation. Things got out of hand, and I’ve really questioned my morals since then.”

Fast forward to this year’s festival.

He had no idea I was going to be there. On the first night, I saw him again… making out with yet another woman.
The next day he had already left the festival to play another gig elsewhere. After he had left, I posted a photo of our campsite on my Instagram story. It was very obvious from the photo that I was at this specific festival—the location was instantly recognizable.
Almost immediately, he started messaging me, asking where I was. He kept asking multiple times where exactly I was. I ignored every message.

I obviously can’t know what was going through his head, but the timing made me wonder whether he suddenly realized I had been at the festival all along and might have seen him cheating the night before.

The thing is, it’s never just a random kiss. He acts like they’re a couple. Holding hands, constantly looking at each other, standing by the DJ booth while he’s playing, turning around to smile and wave at her during his set, being all over each other in public. It’s very much the whole lovey-dovey package.
What makes this even worse to me is that several of his friends (who I have gotten to know over the years) seem to know about this behavior and don’t do anything to stop it. If anything, it feels like they enable or support him.
So from my perspective, his girlfriend is not only being betrayed by him, but potentially also being left completely alone because the people around him are choosing to look the other way.

I don’t believe this is an open relationship, mainly because of how panicked and “caught” he acts whenever he thinks someone might have seen something. Of course, I can’t know their private arrangement for sure, but his reactions make it seem like he knows he is crossing a line.
On top of that, there’s usually a lot of dr*g use involved, which doesn’t exactly make the situation look any better.

So obviously I know the “I questioned my morals” speech was complete BS.

Here’s where I’m struggling:
I don’t know his girlfriend personally, but it literally took me about four minutes on Instagram and Facebook to find out who she is. I know her name, where she lives, what she does for work, and I could easily contact her.

Part of me feels like, if I were in her position, I’d want someone to tell me. Especially because they have a child together.

The other part of me wonders if it’s not my place and whether I should stay out of it.

So… would you tell her? Or would you mind your own business?