According to the website, the fee is waved for Golds and Plats, but not for Silvers . (LINK)

What's interesting, however, is the fact that you are the one who booked the tickets. Since you are the one who were on the phone with the agent, and since the ticket was issued to you (i.e. you paid for it), I don't think it is reasonable for them to apply the fee to your wife's ticket. The issue is the fact this situation is not explicitly mentioned in the T&Cs, and as such it can be interpreted both ways. I would politely explain to them the situation and ask to have that fee refunded, because you have IMO a valid argument. In my experience, NW has always been reasonable.