Sorry for being this tardy in posting a reply - haven't been on in a while. To answer your questions - Texas is fairly friendly to homeschooling:
Homeschools must be conducted in a bona fide manner, using a written curriculum consisting of reading, spelling, grammar, math and a course in good citizenship; no other requirements apply.
In regards to specific curriculum choice, that's largely a matter of personal taste and how close to a single grade level your friend's daughter is. A packaged curriculum will work if she's more or less the same level in everything; mixing and matching may be necessary if she's good in some things and really bad in others. The major homeschool curriculum publishers offer placement testing for identifying grade level, for instance Alpha Omega, Bob Jones, Calvert, Sonlight, etc. There's also the question of learning styles and whether she's visual, auditory, kinesthetic-tactile, has dylexia problems, etc. If you can, attend a local homeschool group meeting and/or homeschool convention and take a look at all the different curriculums people are using.
As for guardianship, you should take that up with the office of the public defender and social services (if you don't have money for a private lawyer, anyway). For purposes of getting guardianship, it'll probably be easier to choose one of the packaged curriculums and say you're going to placement test her and use that, rather than trying to come up with something custom on the spur of the moment. Once she's in Texas and you've had time to breathe a bit, you can look at swapping out any parts you don't like for other subject-specific curriculums (for instance, I like Saxon for math up to Algebra II or so).