I think it is hard to be friends with your boss but I don't think you can't be friends. Dh used to be friends with his old boss and I have invited my boss to birthday parties for my kids and have attended a suprise party for his wife. I think that you have to be aware of that line though and NOT cross it.
I think it can be a fine line, but I have walked it and been ok. I tend to be friends better with males anyway and have always but once had a male boss. The one female one I had was a royal bit*h.