Translation:His face has turned red!
In my experience, yes. This is how i would translate it. (Studied for 10yrs, lived there for 2)
This should also accept "He blushed" and probably "He is blushing" since it's a fleeting change and 了 signifies that.
Not to mention English talks about blushing rather than faces turning red. It is not good English to say, "His face has turned red." It makes sense, but I certainly wouldn't say it that way.
How should 脸 be pronounced? In my dictionary it's spelled lian3, that's the pinyin I have to type to enter the character, but duolingo consistently pronounces it as die3/dian3.
Is this a common mutation, just this word, or is it a mistake in Duolingo?
the listening question does not make sense because of the incorrect pronounciation
This sentence needs 变 or变成 which means turning into. Otherwise you are just saying his face is red
Not according to the dictionaries I've checked which list 脸红 as specifically meaning "to blush" and even to become bashful, embarrassed, or ashamed. Are you a native speaker or just thinking out loud?
了 indicates that a change took place. His face is red now, but it was not red before.
I also respectfully disagree. The 了 signifies the "turning into" enough for me.
That would also work, but I'm not sure how else you'd translate "他的脸红了". Maybe "his face reddened" (which is really the same). Here 了, at the end of the sentence, indicates a change of state; so his face changed from not red to being red.
Basically for any case the face turns red.
1 and 2 are likely the situation. 3 is more often expressed by 气-得-脸-(都)-红了 (都 = adv. even / adv. entirely), and 4 喝-到-脸红. If he is arguing about something: 争-得-脸红耳赤. The corresponding action may also be in another sentence, so the context will tell you more about it.