君は友達には忠実でなければならない。

[MsFixer]

English Translation

You should be true to your friends.

Wrong translation. 忠実 is closer to:

1. “be loyal to someone” – i.e. “You” are the servant of the “friends”, which doesn’t make sense in this context.
2. “to comply with rules/traditions/realities”

You should replace 忠実 with 誠実（せいじつ）or 真摯（しんし）.

• 誠実 means “to be honest and act in good faith with no self interests”.
• 真摯 means “to be devoted to something (including communication with someone) with a pure heart”.