JonTY1, how much does your wife know/care about flying in a premium cabin? (This feels like the bit in the Shawshank Redemption where Andy asks a guard if he trusts his wife and the guard puts him up against a wall...) I ask not to be rude, but because if the answer is 'not much' you can probably hide this by telling a small lie. You could for example, tell her that you are flying WT+ which could explain away the lounge access if she doesn't know the intricacies of lounge access rules.

Alternatively, (heretical for this site) you could skip the MAN lounge altogether which won't give the game away, hopefully, until you get to the CCR.

Lastly and of course this depends from person to person, you could tell her on the night before / the morning that you leave. She may enjoy the anticipating of flying F almost as much as the cabin itself. I know that Mrs Baggageinhall is a lot calmer about the whole travel experience if we are flying F or J rather than Y knowing that she can relax in the lounge, not worry about having to buy extra bottles of water to drink in-flight, having more space etc.