Yes, they can. I would have no problem if my wife had a male best friend.

I have a female best friend that I love with my whole heart. I trust her so much that she's only one of two or three people that I've told about my mental health issues. Earlier this week she moved back home, over 1000 miles away. I'm still having a hard time dealing with it, but we text each other every day so that kind of helps me deal with it.