Yes you can use ONLY your German passport to travel to Brazil, even if you are brazilian by birth.
I just checked the Brazilian Federal Police (which is the authority responsible to issue passports, as well as to do border checks and allow non-immigrants into national territory) website and it clearly says:
Dúvidas sobre uso de passaporte estrangeiro para brasileiros com
Não é obrigatório que o brasileiro com dupla nacionalidade utilize seu
passaporte brasileiro para realizar viagens internacionais.
In a free translation, assuming you know no Portuguese and to a broader audience also interested in the matter:
Questions about brazilians with double-citizenship using a foreign passport:
It is not mandatory that a brazilian with double-citizenship use his brazilian
passport for international travels.
If that is not clear or to the point enough, there is another link with more specific and detailed information about your case:
Brasileiros com passaporte de outro país ,
Apesar de ser brasileiro(a) posso entrar e sair do Brasil com passaporte estrangeiro?
O principal requisito para a entrada e saída do Brasil é a posse de
DOCUMENTO DE VIAGEM VÁLIDO.
O passaporte é o documento de viagem por excelência e não há, na lei
brasileira, restrição quanto à apresentação de passaporte emitido por
outro país quando da entrada ou saída nos controles migratórios.
Brazilians with passport from another country,
Even if I'm brazilian, can I enter and exit Brazil using a foreign passport?
The main requisite for entry and exit from Brazil is having a VALID TRAVEL DOCUMENT.
The passport is the most common travel document and there is no brazilian law that restricts the presentation of a passport issued by a foreign country in entry or exit points when performing border controls.
This last link goes on explaining that it could be a good idea to present any brazilian issued document, even an expired passport, so that the bearer can be determined as being a brazilian with a second citizenship and not a foreigner. For brazilians some immigratory rules do not apply (like having a max allowed stay within the country).
Considering that the Federal Police is an authority on immigration and the last link I posted is fairly recent (2 months old) and clearly states the opposite of what your friend told you, I'd say you're good to come back to the place you were born!
Tenha uma boa viagem!