Is it permissible to make du'a' (supplication) for a person using the form of direct address in prayer?

Question : But if one makes du'a' for a person using the form of direct address, saying, for example, "May Allah forgive you, O Shaykh al-Islam Ibn Taymiyyah," while he is praying, does his prayer become invalid?

Answer : The jurists say, you see, that he used the particle of address, and address is not permissible in prayer, because the Messenger (peace and blessings be upon him) said , and they did not make an exception for anyone but the Prophet (peace and blessings be upon him). They said that he is addressing him, "Peace be upon you, O Prophet," but for anyone else, one does not use the particle of address for him at all. However, there is something in this opinion that leaves one with a sense of unease.
This is because if you say, "May Allah forgive you, so-and-so," while you are praying, you do not feel that you are addressing him at all, but rather you feel that you are keeping him in mind with the utmost presence of mind, as if he were in front of you. It is established from the Prophet (peace and blessings be upon him) that when Satan slipped away from him, he said, "I curse you with the curse of Allah, I curse you with the curse of Allah," so he addressed him. Some say that this hadith was before the prohibition of speech, and some interpret it figuratively. But both answers are subject to scrutiny.
So what appears to be the case is that addressing human beings, for example, if someone passes by you and you say, "So-and-so, come, where are you going?" This is human speech; does it invalidate the prayer or not? It invalidates it. But a person who keeps a dead person in mind and then says, "May Allah forgive you, so-and-so," there is doubt as to whether this is among the speech that invalidates the prayer. To avoid the doubt, instead of saying "May Allah forgive you," he should say, "O Allah, forgive him," and this is permissible by consensus.