My girlfriend (16f) and I (16m) keep running into a man she rejected almost everywhere we go for the past 2 weeks, what can I do about this?

I noticed this about 3 weeks ago, but it stayed consistent for the past 2 weeks. my girlfriend and I barely meet due to our exams and our busy schedule however we make sure to meet at least for 1 hour every other day. We go on walks and dates outside a lot.

Two weeks ago, this guy came up to my girlfriend to greet her. He called it a "good coincidence" (his words). We ran into him 4 times that same week. At the park, at a fast restaurant, a coffee place, and a nice remote field that flowers started blooming in.

this week again, but this time, it was 5 times. Today was the most recent at an ice cream shop. We don't live in a small town, and this city is populated well enough that it is weird to run into someone from your school this many times. She already rejected him previously, and he keeps avoiding me, too. I didn't even try to greet him once, and neither did he. We both acted like the other one did not exist.

I'm wondering if this is a stalking behavior because I feel very unsettled by this guy. I tried looking him up, and his social media seems very generic, and he does not do anything like no sports, after school clubs, or anything. My girlfriend finds it weird as well because this behavior from him is new. I once said to my girlfriend that I don't like our date getting disturbed, and she said goodbye to him and that she should go. It seems he is doing it on purpose.

What can I do about this?