I'm curious about that myself, because the Bible says we're supposed to love God more than Man. And going to another religion's temple is like wilfully going into a den of spiritual adultery, with foreign men hitting on you while you're married to Christ. The message you send that unbelieving friend could also be one of, "I submit to your religion," or "I don't take mine seriously." But it could also be, "I'm stable in my faith, and you can't shake me." So it really depends on what comes of the occasion, and if you think God, as your husband, would approve.