Not sure where the "15 mph" number came from.

I would ask the OP, does your wife use the top couple of cogs? I might go for both a bigger chain ring and a smaller cassette. 40x51 is ridiculous unless you are climbing mountains. Apparently there are 10x45 cassettes available, which would (very, very) slightly increase top end, while providing closer gearing and still plenty of climbing potential .... which you say she doesn't need anyway.

With a 46-tooth ring and a 10-45 cassette your wife would have decent road gearing and still have the same climbing capacity as a 50-34x11-32 combo which is pretty normal for a road bike used for climbing.

In any case .... 44 or 46 teeth on the ring sounds like a good place to start.