Learn Arabic (Modern Standard): وداعا! أنت صديق جيد.

In Arabic, there is no word for "is" or "are" in simple present tense sentences. You just put the subject (أنت meaning "you") directly next to the information about it (صديق جيد meaning "a good friend").

How do we know this is being said to a man and not a woman?

The pronoun أنت (you) is masculine, pronounced "anta". If you were speaking to a woman, it would be pronounced "anti". Additionally, the words for "friend" (صديق) and "good" (جيد) are in their masculine forms, without the round feminine "taa marbuta" ending.

Why does the adjective come after the noun here?

Unlike English, which puts the adjective first ("a good friend"), Arabic grammar requires the noun to come first. So the noun صديق (friend) is placed before the adjective جيد (good).

Why does the word وداعا end with the letter alif (ا)?

This word ends with an "an" sound, which is a grammatical ending called tanween. In Arabic spelling, when a word takes this specific "an" sound, we usually add a silent alif (ا) at the end of the word to support it. You will see this often in fixed greetings, such as مرحبا (welcome).