Firstly, I agree that it's strange to have the father following her.
I would definitely want to know this family. We mums know too much and our daughters not enough to realise the full implications.
Tell her you aren't interested in trying to control her but that you are the person who loves her more than anyone else in the world so you want to be involved in understanding the dynamic and keeping her safe and empowered.
Tell her how happy you are for her that she's found a boy that she's connected with. This is really important. Then be genuinely curious. I find the techniques of super-silence and active listening most helpful. Don't jump in with answers or trying to fix or control, just tell her you really want to understand her so that you can support her in the way that a proper mum should do and explain that support is very different to control.
How did she meet this boy? What does she like so much about him? How does he make her fell? What's his family like? What do they do together? Why doesn't she invite him back to your house?
Do create boundaries that you feel are appropriate. She's only 15 so tell her you still have legal responsibility for her welfare, regardless of whatever she feels.
I would also say that if she feels old enough to be dating a boy then she is also old enough to have some serious conversations with you about contraception, STD's, consent, etc.
I really feel for you. This part of parenting is really hard.