I was editing a friend of mine's paper and they used the sentence "By writing, she gave outlets of feelings and words to those who needed it, to those who did not have expression before." I felt like there needed to be a semicolon after 'those who needed it', but I'm not sure. Is the comma there correct or incorrect?
I think in this particular case it would actually be a colon, but you're right that it shouldn't be a comma.
Since the part after that you're referring to, "...to those who did not have expression before", is not a complete sentence.
Ie. "To those who did not have an expression before." is improper English because it lacks a subject (Who is doing what to "those"?), you could not separate it by a semicolon.
You could remove the "to" from that part of the sentence, and a semicolon would be okay, or you could make it an appositive phrase and write it like it is below.
"By writing, she gave outlets of feelings and words to those, who did not have expression before, who needed it."