They is technically plural. There is no good singular gender neutral pronoun. So, "he/she", "s/he", "he" or "they" are fine by me; we know what the author means.

Where 'technically' means 'people have used it as both singular and plural but some people like to argue' http://en.wikipedia.org/wiki/Singular_they

Perhaps. Some might disagree with that, but the "gender neutral pronouns" debate is both long and boring. After all, he and she have both been used throughout English writing to denote all people male and female. Certainly a person using he/she his/her is well aware of the possibility of using they their instead, and perhaps said person is making a stylistic choice.

I do not object to the use of colons, but if I did — insisting on the use of Em dashes instead — and I were to comment that there were perfectly fine alternatives to the colon on every post that used one (such as yours), I might be considered an overly pedantic nuisance.

'One' may be a good choice also, although singular they also works.

"Maybe one mentors a child in one's spare time."