That is our case, we both love the fantasy and enjoy roleplaying it often. But she is so shy and conservative I just don't know if and when she'll want to take the next step.
Moss wives are often hesitant when it comes time to take a bold step forward. However, I'd suggest most times it works best when she has someone whom she can connect with. A lover who'd come onto her more as a friend than the mere thought of wanting just to fuck. Every wife deserves someone whom she feels can relate better with becoming a worthy sexual substitute to her husband.
Once her lover is about to seduce and conquer her mind, then her body is bound to follow.