My daughter, looking down, saw him put his hand on her friend's knee. The friend, embarrassed, shifted sideways. The man put his hand back on her thigh.
My daughter stood up. "Everyone get up," she said to her friends. Then she turned to the man. "You are disgusting. What are you doing touching up young girls?"
He tried to laugh it off.
"No," she said. "It's not funny. You have no right to do that."
By now all the other people waiting for the bus were watching. Two young men were standing alert as if they might intervene.
The man looked up at my daughter's friends. "She's mad, right?" he said. "She's getting upset over nothing."
A bad miscalculation. They all rounded on him. "She's our friend," they said. "Of course she's not mad. She's right. You're disgusting."
He was still trying to act as if nothing had happened. But the fight had gone out of him. Now he shrank into his coat, sad and wizened like an old walnut.
There's been a lot of talk this week about whether girls invite violence by the way they dress and behave. It all started with the Telegraph interview with Joanna Lumley, who wanted to pass on some advice to young women.
"Don't look like trash," she said, "don't get drunk, don't be sick down your front, don't break your heels and stagger about in the wrong clothes at midnight...because somebody will take advantage of you, either they'll rape you, or they'll knock you on the head or they'll rob you."
I love Joanna Lumley. Patsy is, after all, Absolutely Fabulous.
But I'm so sad she said this. We're back to the old myths that it's a girl's choice of clothes, or drinking too much, or wearing cheap shoes that lead to violence and sexual assault.
Personally, I think it's time to put the blame where it really belongs – with the few sad, disgusting men who need to re-think their attitudes.
Does this sort of behaviour from men make you furious?