Should a friend be informed of a price increase when buying a computer for him, knowing that the seller is aware of the increase and the buyer is not, and the broker (the original purchaser) bears responsibility for modifications and problems?

If your friend appoints you as an agent to purchase a computer, you are his representative and a trustee. It is not permissible for you to take an increase on the purchase price without his knowledge; otherwise, you would be a traitor and would be consuming his wealth unjustly. However, if he asks you to purchase it in your capacity as a merchant, then you buy it, take possession of it, and then sell it to him at an increased price, this is permissible.