I'm a "giant" at 5'6" but my seats sit quite low - the top of the side bolster is even with the sill. Even so, Most of my head is below the top of the windshield. There is a bit of space to raise the seats, if necessary, by sitting the seat rails on top of square steel tubing. Doing that, you can almost raise in 1/4" increments.
Anyway, as most mentioned, I doubt that she'll have any seat trouble with a Speedster.
Now, keeping her right foot off of the gas pedal - That might be another problem!
I also agree that it is really important that she drive it before you buy. It is NOT like driving a Mazda Miata, for sure, and seems to be very much an acquired taste for women, even as passengers.