I think I understand how you feel. I talked my SO into joining and felt somewhat relieved when he cancelled his account after a week. There is nothing that I’ve posted that needs to be hidden from him, but some of the responses I’ve posted just haven’t come up in our personal conversations before. There are topics, like past boyfriends, that I’d prefer he heard from me rather than reading it on this site.
As for what you should do, there doesn’t really seem to be a good solution. An alternate account may work, if you use it for questions and answers that the friend won’t peg as yours from the details or identify as you from your personal style.