"My friends stopped drinking."
Translation:Mis amigos dejaron de beber.
Often, Duo gives "correct" answers in the feminine but will also accept masculine answers as well, if you're translating to Spanish from English. That's more just to show you that you can do that than that there is some "correct" gender involved. Only when gender is apparent in the English (using "she" or "the women" or something like that) do you have to worry about the difference between amigo/a.
In this case you don´t need to think how the sentence would be translated without de, it's just the way it is. In English when you say 'Someone + to be + going + to' you will never consider any other preposition except 'to'. The same thing is with the verb dejar, you just have to remember what preposition with what verb to use.
Some examples you can find here http://users.ipfw.edu/jehle/courses/VRBSPREP.HTM