RHS garden Rosemoor Devon,not that I've ever been but would love to, Plantworld gardens Devon yet another I'd love to visit rare and unusual plants from all over the world, I buy lots of seed from them:-))
http://www.rhs.org.uk/gardens/rosemoor
http://www.gardenvisit.com/garden/plant_world_gardens

One garden from me, The Garden House, Buckland Monachorum, Yelverton, Devon, PL20 7LQ.

http://www.thegardenhouse.org.uk/

Thank you so much everybody, I knew I could count on people on this site to suggest some good places! I will pass all these on to my friend, I'm sure she'll be very happy! Much appreciated :)

How about Winsford walled garden in Devon, owned by Muddywellies (well he's called that on GOY). It looks lovely - there are pictures on this site.

Thanks Cammomile, good idea!