Lewis Carroll: "'Whoever lives there,' thought Alice, 'it'll never do to come upon them this size: why, I should frighten them out of their wits!'"
C. S. Lewis: "She kept her head and kicked her shoes off, as everybody ought to do who falls into deep water in their clothes."
Doris Lessing: "And how easy the way a man or woman would come in here, glance around, find smiles and pleasant looks waiting for them, then wave and sit down by themselves."
E.B. White: "But somebody taught you, didn’t they?”
This is settled. Singular 'they' is good English and always has been. The rule against it was just made up by 18th century grammarians (including the fascinating Ann Fisher [4], who surely would regret it now) and they even got a law passed in 1850 prohibiting it, which only goes to show how widely used it was.
1. https://web.archive.org/web/20150328135337/http://www.crossm...
2. http://itre.cis.upenn.edu/~myl/languagelog/archives/002748.h...
3. http://www.pemberley.com/janeinfo/austhlis.html
4. http://www.nytimes.com/2009/07/26/magazine/26FOB-onlanguage-...
"Alternatively, they as a singular pronoun can refer to an individual without strictly giving it a gender."
https://en.wikipedia.org/wiki/Singular_they
"Singular they is the use of they, or its inflected or derivative forms, such as them, their, or themselves, to refer to a single person or an antecedent that is grammatically singular. It typically occurs with an antecedent of indeterminate gender"
"One" works in some cases. Unfortunately, not when referring to something directly. I'd love to see "onesie" used for that, but it's not going to happen.
If nothing else, one could use "one" to establish singularity when they use "they" later on.
Using "her" actually apparently attracts attention away from the content. Perhaps mixing both within a longer body of text is a more subtle way.
It must be a female.