Lots of dogs, both male and female, neutered and not, will hump other dogs, people, objects, etc. My understanding is that is is a form of excited play, as well as a show of dominance. So in other words, your friend's dog just got a little too excited!! There's really no sexual meaning in what she is doing, but if it is upsetting your friend, she can discourage the dog from that activity with a firm "NO" or other such command.