I haven't bought or used a strap-on for years - double-ended dildos are much nicer, for all kinds of reasons. No irritating straps to spoil the intimacy, or chafe you, and because you are sharing the same object between both your vaginas (or vagina and anus), you can share and feel each other's bodily responses so much more acutely.
The downside is that the absence of straps means at first you feel it's unstable and prone to slipping out of the 'wearer'. There is a bit of a knack to it, but it's well worth persevering.
For lesbian sex, I would always recommend the Tantus Feeldoe range. My ex-girlfriend and I bought several, but our favourite, and a good one for beginners, is the Classic:
I can testify that this toy delivers fabulous orgasms for both parties! It's also really good for anal sex, although the wearer usually needs to hold and guide the shaft manually until you achieve stable and fairly deep penetration.
The real selling point of Feeldoes is that from my experience they are the best designed for the female anatomy, at both ends: the shapes and curves are just right, when 'wearing' the ridges work really nicely against my clitoris, and the colour and design aren't too phallic, which I like.
Now I'm single, alas, I sometimes masturbate with a Feeldoe, and it's good for that too - this is definitely a versatile range.
Another decent model is the Toy Joy - I have one of these too but never seemed to use it quite so much.
Hope your friend and her girlfriend have fun!