Is it Islamically obligatory for me to inform my friend, from whom I bought a device, about the defects it had when I used it, and to tell him that he has the option of returning it due to a defect, even though he might have discovered them but refrains from contacting me out of shyness?

Concealing defects is a form of deception and fraud. You must disclose any defect in the device to the buyer, even after the sale, which then grants the buyer the option of returning the item due to the defect. Whoever desires blessings in their sale should be truthful and reveal the true nature of the commodity and its defects, for the Prophet, peace and blessings be upon him, said: "If they are truthful and make clear, blessings will be bestowed upon them in their sale; and if they lie and conceal, the blessing of their sale will be effaced." Let not shyness prevent you from being truthful, in order to avert suspicion and to ensure the blessing of the sale and the continuation of harmony.