友 alone means friend/companion, and it still means that when written as 友達, but what's the difference?
Answers (1)
You'd be better off asking in a jap learners' forum for subtleties.
For one thing, pronunciation (let's not forget onyomi, kunyomi & nanori) - I've seen that with a few other kanji. 友 may also translate to comrade / pal per my dictionary, if that counts for anything.
This has some interesting hypotheses, if not necessarily pertinent: www . quora . com / In-Japanese-why-do-some-words-use-multiple-Kanji