It is permissible
for the relatives, friends and Muslim brothers and sisters of the deceased to invoke mercy for them and recite other permissible supplications. They will be rewarded for doing so. This is indicated in Allah's saying, (Part No. 24; Page No. 226)
And those who came after them say: "Our Lord! Forgive us and our brethren who have preceded us in Faith, and put not in our hearts any hatred against those who have believed. Our Lord! You are indeed full of kindness, Most Merciful.
There is no special Du`a' that should be said in such cases.May Allah grant us success. May peace and blessings be upon our Prophet Muhammad, his family, and Companions.