Yes, a lot of women with narrow hips do prefer men's saddles. My husband actually has a Terry Fly, and the main differences I see between the Fly and the Butterfly are that the Fly is narrower throughout, and also is a little bit longer.

The Terry Damselfly is also narrower (and again, longer than the Butterfly); although I haven't ridden one, when I held it in the store, it did seem a bit harder than the Butterfly and the Fly (actually in shape and hardness, it was very comparable to a Selle Italia Flite). I have tried a Flite, but after a few hours on the bike, it seemed rather hard and uncomfortable - however, the new gel ones are much better, according to my husband.

I would suggest trying a narrower man's type saddle for your friend. I really like the Terry Butterfly and Fly, but as you know, everyone's rear is different, so what works for one person may not work for another. I can only offer suggestions, the main one being that since your friend is slender with narrow hips, she might do better with a narrower saddle rather than the so-called "women's" type seats.