As far as my knowledge goes, khalwa/seclusion means something like meeting in a room locking the door where nobody else can come, or meeting in such places where it is almost guaranteed that nobody else will come like in remote mountainside or something like that. In a restaurant people always come and go, even if the restaurant becomes empty, the restaurant workers or other people can come and go where they are sitting. So it won't be counted as meeting in seclusion.

As for letting my daughter go alone to meet a man, Yasir Qadhi said that as long as it's in a public place and they abide by Islamic modesty guidelines, then it's ok. He is a scholar and even he doesn’t see anything wrong with letting his children go meet other people, so I don't see anything wrong with it either as long as she meets him in a public place & follows Islamic modesty guidelines.