Hi Eagle
Your friend and I share about the same measurements.
It depends where she will be wearing them really. I am a Medium but have had to buy Large in the past.
The Mediums fit me best (of course) but the large is not "too big" on me either, but there are differences.
If she plans on wearing outside = Definitely get the Mediums

If she only plans on wearing inside then it doesn't really matter.
You will find that the Large will have a taller rise (It will hit higher up her back/stomach) then the Med.
The Large will probably be a bit wider in the crotch and (I guess depending on the brand) be a bit thicker too.
The top (Front and Back) side panels, where the tapes are will be wider in the Large and the tapes will hit more towards the center then the ends.
But as long as both are put on snugly (which should not be a problem with either size) you should have no (extra) problems with leaks.
With Larges I will usually fold down the extra height (where there is no padding) inside the diaper to lower the rise a bit and this does not effect leaks.
Also the larger diapers will cover a bit more of her butt, lol. Not that the med don't cover it, the larges just are a bit wider there too.
The Mediums will give her more of a smaller fitted look, while the larges sort of let you know in no uncertain terms that you are wearing a diaper, lol.
Mediums will look "cute" on her, lol, but if you or her want a more AB look and feel then get the larges.
Hope this helps.