Her options are:
1. enter as a tourist -- she can stay for up to 6 months, but will need to show that she has financial support during that time. She cannot work (or volunteer), stay here longer than 6 months, or enrol in university during that time.
2. enter on a student visa -- she'll be able to work up to 20 hours per week as a student. Note that a student visa is not for permanent settlement -- when she's finished studying, she'll need to leave the UK unless she secures a work permit or is engaged/married to a UK citizen (with the appropriate visas/paperwork) at that time. She'll need to look into financing her studies as international students pay higher fees.
Basically, you don't come into the equation at all. You aren't related to her. You can't adopt her or be her guardian or employ her. You just happen to know her -- it's her job to find a way to get over here (see options 1 and 2 above).
Again, I ask, why are you so desperate to get her over here?