Hi, I'm Dr. Deb. I will do my best to assist you today.
I'm sorry for this concern for Sophie. While we typically think of mounting as being exclusively male behavior, females can engage in it as well and it's perfectly normal behavior.
There are several different reasons why dogs will hump or mount:
I suspect that this is sexual for Sophie since she consistently humps the one "toy".
I wouldn't be concerned about it myself, but if it bothers you, then preventing access to her Boyfriend should solve the problem.
I had a neutered, male cat some years ago who did the same thing but only to one particular toy. It didn't matter if that toy had been out of his sight for months, as soon as we took it down from the shelf where we kept it, he would mount it....and I have to admit that it was pretty amusing:)
I hope this helps.