Well, just now (09Jun2020), the app counted my response, "Baineann sé an cóta de," as correct. I got the question as a "Tap what you hear" exercise; I was choosing from the "word bank" (the word "bubbles" or "tiles") rather than using the keyboard; and both "de" and "di" were available tiles. I chose "de" rather than "di," and got credit for a correct answer, but when I saw the "Meaning: He takes the coat off her," I came to this discussion to verify that my selection ("de") should have been "di." Sure enough, the blue sentence at the top of this page uses "di" and the translation "her," but maybe the pronunciation is close enough that the app lets "de" slide in response to a "Tap what you hear" drill.
I took me seven times but I finally learned how to type this sentence. That is all I just want who reads this to know no matter how many times they try in this impossible question they will succeed eventually. If you made it this far you've come too far to quit. God love you all.
I suppose one reason why it is not "he takes her coat off" is that it might not be her coat: "the" coat might belong to someone else, and the sentence does not specify that it is her coat.
Also, "he takes her coat off" could mean that he had been wearing her coat; that is, "he takes her coat off of himself."
Besides, "he takes her coat off of her" would be "Baineann sé a cotá di," right?
Yes, I know that in speech, "an" often sounds like "a," but in this particular audio clip, "an" really does sound like "an" to me; moreover, the written version of the sentence uses "an" rather than "a."