I wouldn’t have replied. I wouldn’t have been very happy about the “tailing” so my reply would have either been a lie or rude.
My dcs were driven to primary school and get a school bus to (massive catchment) secondary school. They are not especially streetwise and weren’t going on trips to town before y7. I drive everywhere and they have no experience of public transport (except school bus, which isn’t the same). When they started venturing out with pals in secondary then I wanted then to have a nice time and deepen those friendships, be safe, and gain independence. Tbh, I was worried they would get into bother somehow, need me, and me not be there. They have been in bother (missed buses, dead batteries, ruffians at the bus station - but they’ve managed). However, apart from the “be safe” objective, the other things need them to be in their own, not supervised, not “tailed”, not helicoptered. They behave differently if Billy’s mum could pop up at any moment. It stops them enjoying themselves. Next time they won’t invite billy. My ds1 had a parent who did this and it pissed me off. I wanted my child to learn how to navigate the train station, or figure out the bus route. She would offer a lift, I’d decline, she’d scoop him up off the pavement anyway. My kid happy to have a lift, I’d rather he learned how to travel independently (which is why I wasn’t driving him myself). She would try to change their arrangements, tell the, to hang around local small town instead of going to the city with the big skatepark and better food. She’d go shopping in city, trailing around he reluctant daughter. I’d be getting endless messages, even when the boys were 16. They are off to uni in September. Hers is going to London. He’s still never been on a bus. Yes, he’ll pick it up but my parenting is more gentle, age appropriate, challenges starting in babyhood, hers is cotton wool to baptism of fire. I don’t want to have to substitute her parenting for my own. I’d be irked that your actions were impacting on my child (admittedly not that much, and not compared to your own child) and that I was expected to be grateful for it. What am I supposed to say? “Thanks for tailing my kid! Was worried he would have to learn independence!” Smiley face, smiley face, crying with laughter face. I wouldn’t mean it, I would mean “what the fuck”. So I’d say nothing