Formal:
* Adeus, meu amigo/minha amiga. (Goodbye, my friend.) - This is the most formal way to say goodbye.
* Até breve, meu amigo/minha amiga. (See you soon, my friend.) - This is also formal, but implies you'll see each other again soon.
Informal:
* Tchau, amigo/amiga. (Bye, friend.) - This is the most common and informal way to say goodbye.
* Até mais, amigo/amiga. (See you later, friend.) - This is a bit more casual than "Até breve".
* Valeu, amigo/amiga. (Thanks, friend.) - This is a very informal way to say goodbye, usually used among close friends.
More informal:
* Flw, amigo/amiga. (Short for "Falou", meaning "you talked") - This is a very informal, slang term for goodbye, often used among young people.
* E aí, amigo/amiga? (Hey, friend?) - This is used as a greeting, but can also be used as a farewell, especially among close friends.
Remember:
* You can use "meu" (my) for male friends and "minha" (my) for female friends.
* You can also use the shorter forms "amigo" and "amiga" without "meu" or "minha" when the context is clear.
* The formality of the goodbye should be appropriate for your relationship with the person.
I hope this helps you say goodbye to your friend in Portuguese!