Checking multiple sources online, zehn is pronounced like 'zeen' as in the English 'seen'. A friend who has been taking some German lessons at the GI is learning it as 'zaen', as in the English 'zen'. I'm a bit confused now, is 'zaen'/'zen' the official way of pronouncing zehn?
You can find the pronunciation of every German word in Wiktionary, for example:
»zehn« in Wiktionary.
There is a section with the title Aussprache (pronunciation), and there you can find the IPA-Code for it. In case of zehn it is:
in other dictionaries you will find:
[ʦ] and [t͡s] are two ways to write the same sound. This sound is the standard pronunciation of the letter Z in German (and it also is one of the two standard pronunciation of the letter C). It is a t quickly followed by an s like "ts" in "cats".
In English, this sound is rare at the beginning of a word.
You only have it in tsunami = [t͡suˈnɑmi] (when you pronounce it with a clearly audible t at the beginning)
This is a long vowel, and it is rare in English. In Indian English you use it in the word play for the letters ay: [pleː]
If you have problems pronouncing it, you can use [ɛ] like in bed = [bɛd] instead (also: let [lɛt], men [mɛn]). But in »zehn« you need it long, not short. (The sign ː is a length-marker. It indicates, that the preceding vowel needs to be spoken long. Short vowels have no extra sign, so a missing length-marker means a short vowel.)
Using [ɛː] instead of [eː] is not 100% correct, but everybody will understand you.
The vowel [eː] lays half way between [ɛː] and [iː], but you never must use [iː] (like in English see = [siː]) instead of [eː]! Never! If you do it you either get a different word, or a non-existing word, and you will not be understood. If you pronounce »zehn« with [iː], you get [ʦiːn], which is the single-syllable way to pronounce the verb »ziehen« (to pull). (The two-syllable-pronunciation of ziehen is [ˈʦiːən], which is considered to be the standard, but you will hear [ʦiːn] almost as frequent).
This is the standard pronunciation of the letter n in English and in German like in the english words tin = [tin] and nice = [naɪs].
German native speakers very often use colloquial speech, or they use a dialect. There are thousands of different local dialects and also the specific shaping of colloquial speech varies in a wide range across the German spoken area.
So when you listen to native speakers when they talk with each other, you may hear many different variations, depending on the region where you are.
You should just know, that there is a really big amount of such variations, but they are not standard German. When you learn German as a foreign language, you should learn and use the standard. You can begin thinking of learning a regional or local variation, when your standard German is perfect. Perfect means: without any accent.
Even the standard of German has variations! There are three standards: German German, Austrian German and Swiss German. They are very similar, but not identical. (The situation is like American vs. British English.) If you plan to live in one of those countries, learn the standard of this country. If your not planing to move to a German spoken country, learn German German. 90% of all German native speakers speak it, and the other 10% will understand it.