Bear with me

So I went on a daytrip with a friend group of mine which included 2 girls. I put pictures on my Instagram story, you know the drill. A day after the trip this random guy that I haven't heard from in 8 years got my number from a friend of mine and sent me a message (told that friend who gave out my number to never do that again without asking me). The guy who sent me a message told me he was going to ask me something but wants me to keep it between us. Anyway, he asked me about 1 of the 2 girls and if I'm serious with her. But don't get me wrong, this guy I'm talking about isn't like this agressive dude who tries to knock you out for speaking to a girl, he was genuinly curious.

Knowing the guy in the past I just replied that I'm not looking for anything serious with the girl and that we're just a friend group. Then he asked me if I hang out with her alot. At this point I wanted to tell him to fuck off and that it's none of his business, but I politely said "nop". Then I asked if something's wrong. He replied no. I asked if he's looking for something serious with the girl and once again he replied no, but he told me he's just talking to her on Instagram. But he also told me she doesn't know him. what??

Long story short. He wondered if I had any information I could share with him about the girl. Which I found weird. I replied no, and that I don't know her that well. He then proceeded to say thanks anyway and I changed subject and started talking about other things. This guy obviously had no confidence and is as beta as you can be.

But the weird thing is, I looked the dude up on Instagram, he doesn't even follow the girl which made me wonder how he's talking to her? I'm seriously not interested in the girl but I was wondering what kind of behaviour this guy has. Is this the perfect example of an orbiter?

Should I tell my friend she's talking with a possible stalker, even if I made the promise to keep it between us with the dude? Because he seemed to know alot about her for some reason. Or should I just let it be, I don't intend on speaking with the guy again.

Feel free to say what you would've done in this situation.