B.C.
I had friends in high school from all walks of life. Some were from some way permissive families, some were super religious, some were very strict (and a few of those had kids who'd sneak behind their parents backs drinking, drugs, sex, etc - parents never had a clue or just looked the other way). Teenagers start seeing and comparing how others live and they begin deciding how they want to be. They see what works, and eventually see what doesn't work. And a lot of the time, by the time they get into their mid 20's they usually pick lives that are not too far from how they were raised.
I wouldn't stop your daughter from seeing her friend, and it's great they spend more time at your house. Teens still want some boundaries, though it's tough to figure where the exact line is sometimes. Talk to your daughter about why you think sleeping over boy friends/ girl friends is a bad idea (just so she's clear you'd not allow such a thing). Your house, your rules is very easy to understand. And absolutely do everything you can to have them at your house as much as possible. There's absolutely nothing wrong with old fashioned. And who knows? Maybe your view will rub off a little on your daughters friend in the long run.