"Stay the night?" I said, hysteria rising.
"Lots of us," he said airily.
"In the same room?" I said, in a strangled gasp.
"Probably," he said. "Although sometimes the girls get fed up and go somewhere else."
It turned out that about 10 of them were staying over and were going to camp in the living room. (I have never met Kate's parents. But I think they are probably saints.)
Since those early years, my eldest has since spent many nights on sofas, on floors and, once, in an empty bath. (Which wasn't entirely successful as someone in the middle of the night, as a joke, turned the taps on.)
The point is that boys and girls sleeping in the same room is not necessarily a cause for concern. One boy and one girl alone in a house or flat probably is, if your concerns run to teenage sex.
But mass sleep-ins shouldn't, I think, make alarm bells ring.
My 17-year-old had six friends – five girls and a boy – over on Friday night. I rushed around in a frenzied sort of way the night before finding sleeping bags and pillows. When we got home at midnight, they had blown up the inflatable mattress, which took up most of the living room, and were lying around watching a DVD, all tucked up with blankets and duvets.
The kitten, purring from lap to lap, was in heaven.
I must admit, as I looked in on them before I went to bed, that I felt quite jealous. I don't think that mixed teenage sleepovers would have been on the cards when I was 17. (I'm not sure my Dad was that keen on mixed teenage parties of any kind.)
But I'm much happier knowing my daughter is safe, with friends, in someone's house rather than shivering at a bus stop in the middle of the night in a part of the city she doesn't know.
Personally, I don't think I'd choose to wake up in someone else's living room covered in crisp crumbs and old popcorn after falling asleep fully clothed at 3am.
But teenagers like it.
What do you think? Do you allow mixed sleepovers?