Archaic noun for a friend or companion. In modern Japanese, this word is obsolete; use 友達 (ともだち) or 友人 (ゆうじん) instead. The kanji 友子 is now almost exclusively read as the female given name Tomoko.
In old documents, 友子 can be read as ともこ and mean 'friend'.
The historical derivation is uncertain. The kanji 友 (friend) and 子 (child/person) combine to suggest a companion, but this usage is archaic and no longer productive.