How can one get rid of the feeling of showing off (riya') when completing the Qur’an and holding a ceremony for it?

If the questioner's sister feels joy at the blessing of memorizing the Holy Qur’an, this is not considered showing off (riya’). Holding a celebration does not affect sincerity, because joy in obedience does not contradict sincerity. She must praise Allah and not care about people's praise, for people's praise for good deeds is the immediate glad tidings for the believer. However, if her feeling is truly ostentatious (riya’), then she must strive against herself, repent, and frequently make the supplication: "O Allah, we seek refuge in You from associating anything with You knowingly, and we seek Your forgiveness for what we do not know."