It kind of depends how much board swapping you want to do.
The easiest option (and perhaps the best) would be to stick with the Viper for now and learn the basics on that. The Viper is a great board and people are often too quick to progress onto a large (140-150) Freeride board when they would make more progress by sticking with the bigger board and daggerboard.
Light wind skills are essential - without these, progress in windsurfing will be a lot slower - the Viper is perfect for this.
We actually did some testing last year of 145 Freerides vs Viper/Starboard Go style boards. Suprisingly, whilst the Viper feels like it should be slower, there is very very little in it for speed (planing).
The Viper was a little slower to plane than the freeride equivalent (but not a lot in it).
The speed sensation is numbed down (because of the heavier weight), but in some ways this is a good thing as the board is more tolerant of clumsy technique.
Our advanced clones were quite happy carve gybing the viper (and found not much dissadvantage over the freeride board) and even managed a few forward loops on the Viper just to prove its not only suitable for beginners!
Progressing to a 'more advanced' freeride board might seem like a forward step, but our opinion would be to stick with the Viper and your wife's skills and confidence are likely to progress a lot quicker.
When she is ready (having a good go at carve gybes and comfortable in straps and harness) then maybe a smaller 120/130 would be a better progression as this will match her weight/sail sizes better.
You could change to the 140ish now, which will make the ride sensation a bit more exciting and make the board more responsive (not to mention easier to carry to the beach!), but once she is starting to try carve gybes, she will probably want a smaller board and be careful not to miss out on the light wind stuff that is so important at this stage (tacking, sail spins, heli tacks, flare gybes etc etc).....
Good luck!