Is it permissible to perform Umrah on behalf of a friend's father after having already performed Umrah for oneself?

As long as you have performed the obligatory 'Umrah for yourself, there is no impediment to performing 'Umrah on behalf of someone else, based on what was reported from Ibn 'Abbas (may Allah be pleased with them both), that the Prophet (peace and blessings of Allah be upon him) said to someone who intended to perform Hajj on behalf of another before performing it for himself: "Perform Hajj for yourself, then perform Hajj for Shubrumah."