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My friend says this English is wrong

Artists at the workshop, one of them who is an award winner, will demonstrate innovative painting techniques that allow painters to create three-dimensional paintings.

But he says the following is correct,,
Artists at the workshop, one of whom is an award winner, will demonstrate innovative painting techniques that allow painters to create three-dimensional paintings.
I' confused .I like the first one...is that not correct English? Need help .What is your take on this ?
Could you please elaborate.
Comments  
Your friend is right.

Artists, one of whom is an award winner, will...
Artists, one of them an award winner, will...
Two artists who are award winners will...
Also, "two award-winning artists" would work.
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Grammar GeekYour friend is right.Artists, one of whom is an award winner, will...Artists, one of them an award winner, will...Two artists who are award winners will...
I could not get you.

Also you missed "is" here

"....one of them IS an award winner..."
Adding the word "is" would make that a complete sentence, which is undesirable in this case. Colloquially, it is acceptable to use the phrasing he chose; I, however, will concede that it is grammatically unstable.

The problem with the original sentence was that you described the object of a preposition with the subjective pronoun "who." That is why you must use the word whom.
so, in the first sentence we can reduce the
them + who = whom

Is it ok ?
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I'd recommend doing that.
Just to be clear - your original "them who" is in an incorrect sentence. Using the "one of whom" construction solves that.

(Additionally, there is nothing "grammatically unstable" about the second formation: Artists, one of them an award winner, will.... )