I'm an old school Eurodollar spreader. A fly is 1:2:1. He can use TN in place of ZN to have a better duration dispersion. If you decide to change the ratios, I'd recommend that you trade them as 2 separate spreads.
If you have a halfway decent spreader, you should be able to trade them in any ratio you like, I suppose... And yeah, I agree, it would make quite a bit more sense to use the new TN as the belly of that fly to make it a little less skewed. But then everything would look a LOT different.