Duolingo has taught that grandmother is bibi, but a native speaker has told me that this is incorrect and bibi is actually the word for wife. He says that nyanya is the correct word for grandmother. Please could someone clarify what is going on, why would duolingo have this wrong?
both "nyanya" and "bibi" can mean grandmother, and "bibi" can mean wife as well:
the fact is there is a lot of regional variation in swahili, and your native speaker friend might have just come from a place where "nyanya" is used for grandmother. if you want to speak the language, you should learn all the meanings of both terms.