There is no need for a knife.
You need an anchor, a 5:1 mechanical advantage lift with an autolock of some kind and a releasable knot (muenter/mule or mariners) to her harness, and a second line (either your spidered rappel or an independently belayed lowering line or even two) to her harness.
Build the anchor, prerig the releaseable knot, build the 5:1, pick her up to unweight the knot to her harness, secure her to either your spidered rappel or tie her into both lowering lines (redundant because this is a rescue), untie her original knot, and then slowly release either the muenter/mule or mariner's to lower her onto the new line(s). Rappel or lower.
As I'm providing instruction on the intardweb, I must add...climbing is dangerous, try this at your own risk, seek out competent real life instruction....blah, blah, blah. Practice, practice, practice. Don't believe everything you read on the internet.
This stuff can be a lot of fun if you like gear and rigging. Every once in a long while it comes in very handy.