According to Some Aspects of the History of Modern Hindi "Nahîn" "No", "Not" by L. A. Schwarzschild, the Hindi interjection nahîn (and Marathi nahi, Gujerati nahi(m), etc.):
is used as an equivalent of "no" (though it may serve also as a negative adverb), and it represents an enlargement of the old Indo-European negative particile, Sanskrit na.
The question asker's name, Raghav, is of Sanskrit origin, so he and his friend are probably Indian. I've not heard nah used in this way, and it sounds like colloquial Indian English. If so, this nah may be well understood when talking to other Indians but not in other countries. It may be that your friend thinks it is too informal and isn't appropriate to be used in a professional context.
You can ask your friend exactly what he thinks, there may be some acceptable uses. I would advise against using it internationally, because it may be misunderstood.
An answer on Yahoo! Anwsers! explains appending "na" turns a statement into a question, and is Hinglish (or colloquial Hindi English), and can also be used like the English "right?":
Adding na in the ending of any statement makes it a question. eg
Statement: "Tu Calcutta se hai" Question: "Tu Calcutta se hai na ?"
Translation Statement :"You are from Calcutta" Question: "You are from
Calcutta na ?"
I use it many times when I chat Hinglish. But be aware using it
formally since this is informal spoken form and not expected to be
practiced in written form except IM's chats etc.
English equivalent for na would be "right"
Eg " You are from Calcutta right ?"
In the statement "Sorry na" speaker expects the reply or an attention
from you "sorry na"could be explained in many ways.
- I'm sorry, Is that ok?
- I'm sorry, are you alright now ?
- I'm sorry, Is it fine with you now ?
- I'm sorry, are you happy now ?
According to a blogpost called Indian English – Read it, na! says na finds its way to the end of many Indian English sentences:
Isn’t the genesis or the logic behind Indian English fascinating “na”? Tell me “no”.. My all time favorite is the linguistic interchangeability of no and na and how it discreetly finds its place at the end of each sentence. Did you laugh, na? Amazing, na? I meant our Indian English.
Jason Baldridge wrote in Linguistic and Social Characteristics of Indian English :
When Indians use English, it is often a mixture of English, Hindi, and other languages. B.C., A.S., and S.Singh called this way of speaking kichiri (2.2.3). Kichiri is a meal which is composed of several random ingredients -- a rather accurate description of the way Indians often talk to one another. Even in "pure" Indian English, many Indian terms slip in frequently. Some expressions such as general mai (in general) and ek minute (one minute) are prevalent in Indian English. N.G. mentions the Gujarati expression take care karje (do take care) in 1.1.5. These mixtures come quite naturally when one is acquainted with two or more languages. ... Her use of nahi (no) in 1.1.2, and S.Singh's use of kya (what) in 2.1.1 are typical of the sorts of ways Hindi terms are employed.
He says, "Yeah, like this guy Gotham felt like when he went back, no?" This use of no (and the expression isn't it in the same manner) stems from the use of na in Hindi, which is exemplified by N.G. in 1.1.6, "...take care karje appli ker hai na?" This could be roughly translated as "...take care karje can be applied, can't it?"
Some more discussion can be found in Google Books, and here's an example in literature.