The Rab Vidda has a long back but I'm not sure if the hood is helmet compatable.
My only gripes with it are
1: The small toggle adjusters on the bottom of the jacket and on the back of the hood.
2: the pockets have an internal storm flap rather than an external one. So the laminated ones are exposed have to be pushed fully shut into a wee hood at the top of the zip. I find it a bit of a fiddle getting into the pockets, especially in gloves.
Otherwise it's not a bad jacket
I'm not sure how tall your wife is, but I'm 5ft 5.5 in and the Vidda easily covers my bum.
As John said some of the ME jackets are longer cut. Especially the men's ones. I had a (now sadly discontinued) Lhotse and the body length was very good. I have just bought a Diamir (womens) and it does cover my bum, just. However shop around- full price it is very expensive- ridiculously so. I managed to find it online with £100 off.
When you're looking online, see if you can find a back length mentioned in the spec. I know when TGO review jackets they give back lengths.
Have a look here for a start to the search- back lengths are given which might be helpful to you.