Many people have no problem [and even prefer friction shifting] however when I got my first indexed bike [too many years ago now] there really was no contest. A bit like buying a car with an old crash gearbox. If as you say she is still inexperienced I suspect she might not thank you for giving her something else to worry about when riding. Indexed downtube shifters are no real problem as the shift is more or less sorted for you, click and there it is, easy, simple and requires no more skill than thet needed to take a hand off the bars [as when giving a hand signal, although I admit not many seem to bother with them anymore]. All the kit is as you have spotted,available on E bay and for old bikes it is cheap. One thought, it may be easier to buy a cheap "donor" bike and simply transfer the parts over.