SATURATE her hair every night with olive oil. Comb through with a lice comb. Use a plastic (disposable) shower cap for her to sleep in (this helps the smother process). Cover her pillowcase/sheets with junky towels (the towels won’t help get rid of lice but will help keep grease stains off everything). Wash it out in the morning (mix dish soap with shampoo. the dish soap cuts through the grease. shampoo alone won’t work). Do that for 2 weeks. Then go every other night, then once a week, etc.
The lice WILL go away (you can tell by combing); make sure you get the nits out too.
If it comes back she’s being re-infested, as popa mentioned.
Recommended to treat siblings and parents as well, otherwise you’re liable to end up on an endless merry-go-round of lice.