Because the speaker doesn't know if the friends are women or not. Freunde can be both male and an unspecified general friends. If you ask Sind ihre Freundinnen Frauen? you would be asking "Are her female friends women?" which would make much less sense. If you know enough to ask about Freundinnen, then presumable you know they are women.
Like how in English, "you" can be one person or several -- how do you know which is meant?
If you say, "Come here!", how do you know whether you are talking to one person or to an entire group?
Only context can help. In the absence of context, both interpretations are possible.
Plural form of you in this sentence would be "Seid eure Freunde Frauen?".
If instead of "her friends," we wanted to say "your friends" "(2nd person: du/ihr/Sie,) the options would be the following: "Sind deine Freunde Frauen?" (deine/your = informal sing. "you") Sind eure Freunde Frauen? (ihr/your = informal plural, as in y'all) "Sind Ihre Freunde Frauen?" (Ihre/your = formal singular or plural, note the upper case "Ihre",) As for the last option, note that Duo's phrase: "Sind ihre Freunde Frauen" (ihre/her = is in lower case which distinguishes it in writing only from the formal Ihre. In the spoken language you'll need to put it in context as both ihre/Ihre are pronounced the same way.)
Note for razorwynd, English Grammar: re "your guy's" - is wrong. The correct colloquial/regional form is- you guys - where "guys" is always plural Note that the possessive form of 'you guys' = 'your guys's,' is considered very, very bizarre, not to say, wrong. More info on this link: https://english.stackexchange.com/questions/12277/what-is-the-possessive-of-you-guys
No, they're not interchangeable. "Women" is a plural noun. "Female" is in this context an adjective. "Are her friends female?" would be Sind ihre Freunde weiblich?
Even if you consider "female" to be a noun (which sounds a bit strange to me in everyday speech about people), then your sentence would need to be "Are her friends females?". This is not however an accepted answer, as it sounds weird in the context of people. If talking about animals however, you might use the word Weibchen for "a female (animal)".
it was wrong.
What was your entire answer?
Nobody can see what you wrote, so talking about "it" or "this" or "my answer", or only quoting individual words from your answer, are not helpful.
Ideally, take a screenshot that shows the question and your answer, upload it to a website somewhere (e.g. imgur) and tell us the URL to the image, so that we can see exactly what you wrote and also what kind of exercise it was (e.g. translation versus type what you hear).