It's a great idea.
Depending upon her size, a shortscale bass might be better for her (get the case and strap thrown in for free), get a 20W practice amp, some good leads, tuner and some overear headphones, plus a 3.5mm jack lead to be able to play tracks through into the amp. Find out about good posture now, rather than let her get into bad habits - the number of people I know who play like Paul McCartney look dodgy as hell, but the number who complain of wrist tendonitis or back pain, but swing the thing round their knees and play like an ape are far greater. In comparison, I play despite arthritis and can still play even in a flare because I paid attention to posture sooner.
TAB is easiest.
She will have to use her ears - never trust anything on the internet that says 'it goes like THIS'. It never does.
Searching for Isolated Bass tracks on YouTube is interesting, but the stuff they isolate on is often beyond beginners.
The learning curve to play bass is much shallower than on guitar, so she will be competent quicker. Being able to play guitar as well is great, but not obligatory - I'd definitely recommend learning music theory from the start, though, as even knowing what three notes comprise a major or minor chord is really helpful in working out basslines.