The hardest part relative to making a story for a team is balancing the encounters, especially if you start at level 1. I would recommend starting a little higher, like level 3,so you can have a variety of monsters without threatening to kill the character outright at first level.
The story itself should be easier, since you can custom tailor it to the one character without having to tie together a group with disparate wants. Just have her come up with a backstory and motivations first, then based on that come up with something the character would find compelling. Try to avoid a campaign that "forces" the character into action; it feels more natural and organic if the character would legitimately want to do the quest.
Once you have that initial motivation, adding more characters is easier than starting with them. This is because the new characters can be made with the goal in mind that they will also want to be on this quest.
Make your starting setting a small town. With only one PC, she's going to need local allies, but having to populate a city from scratch will be overwhelming for a GM.
Have her PC run into an NPC who already has a mission; she can help them with their quest, and later on, they can return the favor. Rescue missions are always good, as she'll have her rescuee tagging along on the way out to help with encounters. If she needs healing, let her meet a cleric at the local church who can later employ her; spell-casting, same thing. Trading favors works in lieu of payment and moves the story forward ("I will cast this spell for you; in return, go out and find out why the shipment I'm expecting hasn't arrived").
Keep the quests short and local at first; as she gets higher-level, she may go farther. Have a big city within a couple of days' travel for shopping, further quests, etc.
Have a random encounter table for the area; then, when you roll one, think about why that encounter is there. Random encounters can turn into major storylines when it's not just 1d4 bandits but 1d4 bandits who secretly work for an employee at a local inn who knows which travelers are wealthy, what they're carrying, and when they're leaving town.
Contests are always fun: a tournament, a joust, an archery contest, a magic show. Whatever her PC is good at, give her a way to test herself against other local specialists. She may make friends (to go questing with later) or rivals. It also gives an anonymous character instant notoriety to place well and explains why a nobleman might employ her ("I saw you win the contest last week; if you can be discreet, I have a job for you").
My wife and I are using the Age of Ashes AP. She is running 2 PCs and I am running 2. So what I do is give her the power of choices. I know she is more of combat player so I give her story beats that make since. She has a map and makes choices where to go. We roll dice and have fun.
https://www.rpg.net/columns/list-column.phtml?colname=duets
Someone is posting reports of their solo campaign on EN World, might be some good tips; https://www.enworld.org/threads/skull-shackles-2e-conversion-solo-campaign. 672607/