Am I obligated to return the profit to the buyer, or bear part of the loss, after it became clear that the item I purchased for him and received a profit on was counterfeit, and the seller refused to take it back?

It is permissible for a person to buy a commodity for another in exchange for a known fee or a percentage of its price, because this falls under the category of agency for a fee, and that is permissible. If you warned your friend that the commodity was not original and he insisted on buying it, then nothing is incumbent upon you, and you are entitled to the agreed-upon profit. If you voluntarily waive or reduce some of the profit to console him, this is an act of kindness and benevolence.