どんなｖｓなんという
 

I was writing to a friend and to cut it short I was kidding and said "what kind of brother are you...". So I wrote どんな兄ちゃん...and he said the correct way to say it is なんという兄ちゃん...
I am a little confused. I checked the dictionary and for なんという it gives examples like "how beautiful" or similar...
Any help is appreciated.

What was the whole phrase you used?

What matters is how you end the どんな兄ちゃん phrase. It is completely correct to use "どんな兄ちゃん + other words" but choose the wrong other words and you will destroy the whole sentence.

By the use of the word 兄ちゃん, I am assuming that you are pretty close to that person's younger brother or sister. You could have said :
どんな兄ちゃんだよ！
どんな兄ちゃんなんだよ！
The ending could NOT be made more polite because of the use of the informal 兄ちゃん.

You can say the same thing using なんという.
なんというお兄さんですか！
なんというお兄さんなんですか！

All these sentences mean "What kinda big brother is he?" or "Some kinda big brother you've got!" These are said humorously after hearing something unusual or weird about the big bro.

Finally, your dictionary is wrong in saying that. なんという can be used at least in two ways and the meaning changes with the wording around it.
これはなんという食べ物ですか。 Asking for the name.
なんというきれいな湖だ！ Exclamation.

I was really looking forward to your explanation, and you didn't disappoint. I didn't think that どんな〜 was wrong, but was confused since it was a native saying so.

Relatedly, does どんなやつかよ work for "what a bastard" or something like that in reaction to seeing some guy be a jerk? I'm trying to remember if I've seen that before.

I was confused, too, because we would use どんな considerably more often than なんという to say what OP seems to have wanted to say.

It works if you replace the か with だ.

It is either
どんなやつだよ！ or
どんなやつなんだよ！

merci beaucoup

Sorry for omitting the rest, but I didn't think the difference so subtle. Yes, the phrase had a sarcastic/joking tone about the brother not helping him.

なんだ。どんな兄ちゃんなのか。彼を助けなかったね。

This is how I wrote is and he told me I needed to use なんという