I agree with Ryokoさん. If I say etymologically, とも(friend)＋たち(plural)→ともだち. It's和語<わご>／大和言葉<やまとことば> words of Japanese origin. And if we try to write it in kanji, we have to write 友だち. たち is a suffix. There are no kanji to express the grammatical meaning. But our ancestors made an effort to write the whole word in kanji. I think the reason is because they wanted to attach the unity as one word. So they borrowed 音読<おんよ>みsound from Chinese of 達<タツ・タチ・ダチ>. 達 means to go smoothly. It does not mean "plural". It's a 借字<しゃくじ>／当て字<あてじ> phonetic equivalent / substitute character. Many people write ご存知ですか. If we write etymologically, it should be 御<ご>存じですか. ゾンジcomes from 存じます. じ is a conjugative / conjugational suffix. 知 is also an当て字. There are another word 存知<ゾンチ>(knowing / being aware). We can also think that it influenced / affected ゾンジ. I prefer 友だち(and 存じ).

In the school, especially in the examination, we have to use the kanji we've learned. In the formal document or something, too, we need to use kanji. However, I think, in our daily life, there isn't specific rule. We can choose whatever we like if writing in hiragana / katakana doesn't confuse the meaning. For example, I prefer writing 友だち because I like this way and it seems to be soft TO ME. It depends on the person. I hope other people give you their opinion.