Letting her succeed on 5+ would be a good idea, in my opinion. But then I think that should be done in any game, not just a one on one =)
I agree, but can you explain a bit more about why you think that? What are the reasons and the consequences? I imagine that would mean less need to push rolls and so fewer WP.
Good point about the WP, it should be said it was the first thing our group changed from the basic rules, so instead of only gaining them through pushed rolls, you have a base pool of (INT+CHA)/2 rounded down (or what ever the two mental attributes are called). In addition to this, WPs are earned in the usual way, but they reset whenever you undertake a long rest.
This might be something to consider.
As for the 5+, I haven't been able to convince my GM about this (he likes it when we fail), but for this one on one game I feel it would make the character a bit more bad-ass (I would consider leaving successes to 6:s for enemies).
It would reduce the need for pushing rolls which means less damage to attributes (the character will be able to do more stuff, for longer without having to stop and rest all the time), as well as damage to equipment (which i feel breaks to easily anyway). It will simply make the character last longer which, and reduce the need to constantly return to a village to get new equipment.
Especially since a one-character party probably wont have all the neccecart talents to be self reliant when it comes to maintenance.
It will also mean that once a roll IS pushed, it should give more successes per dispair in average, rather than an equal ammount. Again making the character feel a bit more awesome than an average one, and increase the fealing of things going the characters way every once in a while.
It also opens up for a new avenue to indicate difficulty of tasks: instead of just modifying the dice (in a game where the number of dice you get is often quite low unless you can manage to twist every situation to be about the one or two skills you acctually have decent values in), you can change on what number you succeed.
I, who prefer to roll dice pretty often even in minor situations, both as a GM and as a player, would probably make three tiers of difficulty based on this instead:
Easy tasks: succeed on 4+
Challenging tasks: Succeed on 5+
Difficult tasks: succeed on 6:s
I prefer this to "GM decides if you succed or not" as it makes the story a bit more unpredictable for both player and GM, without anyone person having final say (well... the GM still has final say but I hope you get what I'm trying to say here). I think this might be extra important in a one on one game, as fewer players mean less input of ideas and suggestions.