What is the ruling on accepting a gift in a commercial transaction, knowing that the intermediary did not inform the buyers about the additional gift offer, but he did not lie to them?

Your friend is an agent for his friends in purchasing, and he is not entitled to keep what he bought for them, nor what was gifted to him because of purchasing for them, except with their knowledge and consent. For the gift that the agent receives is considered a reduction in price and belongs to the principal. Therefore, your friend must return the gift to his friends, and if they permit him to take it, then it is permissible for him to do so.