I'm assuming this is for HRT purposes and she's post menopausal?
I have a lot of thoughts but without knowing a little more about her health status and what (if any other) hormones she's on, including BC pills, and what kind of doctor she's seeing who wants to do this, I'm flying blind.
The thing is female libido is an orchestra. If the only reason she's on testosterone is libido I'd like to know what's going on with her other hormones, including thyroid. Simply making sure her estrogen/progesterone levels are good can correct a lot. A good DHEA will naturally convert into testosterone in a lot of women (I had to stop taking it, my levels went too high and my hair started falling out). There's other ways to use testosterone that won't be in her system so long if her levels go too high or she has undesirable side effects. The bad side effects of testosterone don't always reverse in women (hairloss, voice deepening, undesired body hair growth).
Don't misunderstand, I'm all for testosterone therapy in women, but only if it's necessary and looked at from a whole endocrine balance thing, and it has to be used with care.