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.
Strangely, I have noticed Bibi is taught to be grandmother at the beginning of the course but occasionally in the review lessons I have found nyanya used in a multiple choice answer (Where English is given and you have to select the matching Swahili phrase and the phrase does involve the word grandmother). Outside of this single question it is never used (Though I am still in the far beginning of the course).
Duolingo has it right. Both bibi and nyanya are correct words for grandmother but bibi is more common. There might be regions that prefer to use "nyanya" within East and Central Africa however you are more likely to find nyanya used in books.
Both mke and bibi can mean wife. Bibi can also be used to mean girlfriend.
Finally, a bride is "bibi harusi" (or "bibi arusi"), NOT "mke harusi" (or "mke arusi").