From my personal experience DMing for my longtime GF and relatively new player:
Games flow fast with only one PC. Even with NPCs and DMPCs, expect things to go at least twice as fast as usual. This is good. Be mindful of your player's pacing, though. If they're enjoying a conversation or battle, don't rush it. If they're bored, move on.
Going to echo >>60376666
Do NOT scale encounters up.
DO NOT SCALE ENCOUNTERS UP
There is one PC, and you are not competing against them. If they die, there is a lot more to lose than there would be in a group of 3-5 other players. Make encounters for her level, make them challenging, but this is not the time to experiment with baby beholders and mini mindflayers. Once she understands the principles of combat, then bring in the big guns. This may mean being totally transparent about levels, what moves are being used, what her options in combat are because of the environment, and so on. Reward creativity heavily. Do not punish her with cool encounters that she cannot solve.
Bad encounter: A magic-resistant superboss in a small room that must be defeated to proceed. Failure means death.
Good encounter: A magic-resistant series of enemies in an enclosure with a dynamic environment. They can be evaded, reasoned with, fought or anything else that the player can think of. Failure may mean capture or being badly injured, but not outright killed unless the player does something terribly stupid.
Please take a metric fuckton of notes. Be consistent! Listen to what your player tells you in and out of character, and build the world up to be BELIEVABLE but fun.
See if she'll let you see HER notes, and base shit off of that (different spelling of a name or place? That's the official name. She interpreted the solution to a puzzle one way? Think about why she came to a different conclusion.)
Most importantly, always communicate.
If you have any other specific questions OP just let me know. I hope you have a very successful campaign!