Like others have said, it's probably best for now to just use she/her in your day to day life since they haven't told anyone else. Just to give a heads up though: most people who use multiple sets of pronouns such as she/they prefer them to be alternated. So for example you would say "she did this" or "they did this" rather than "she/they did this."
Some people prefer to have a mix, so not constantly using the same set (eg only she/her or only they/them) while others are simply okay with either option. Since she would be hearing she/her all day, they might appreciate you using they/them more to balance it out. It could be nice to have a discussion about this with them, and give her the opportunity to try out different things. /r/transtryouts will have some examples of what this might look like.
None of what I've talked about is high stakes at all. Even misgendering on accident (in this case using he/him) is something that you quickly correct and move on from, as you would for a cis person. Hope this helps!
This video is a great discussion of literacy. To put that rate into context, 'illiterate' often includes people that can read and write a little bit, but still struggle with some vital or everyday tasks. According to Wikipedia, 20% of US adults have a literacy level at or below level 1 which would be 80 people in this example. This report has a ton of stats and also defines each level of literacy.