Who cares?

What any person does before a relationship is their business. I have no issue with my partners sleeping with multiple people, but I grew up in the countryside where there wasn't a lot else to do, so maybe I'm realistic about this.

If she has slept with a decent number of people, and she is happy with your friend, he should be happy. She obviously knows what she wants, and she doesn't seem as though she'll having that "what have I missed out on" crisis.

I think it's good for people to do as much living as possible before settling down.