Every innovation in the Deen will not be accepted, and you should not say Ameen to that duas. saying Ameen to what he is saying is like you are accepting what he is doing.
'Aishah (May Allah be pleased with her) reported:
Messenger of Allah (ﷺ) said, "If anyone introduces in our matter something which does not belong to it, will be rejected".
Abu Hurairah (My Allah be pleased with him) reported:
The messenger of Allah (PBUH) said, "Allah the Exalted says: 'I am as my slave expects me to be, and I am with him when he remembers Me. If he remembers Me inwardly, I will remember him inwardly, and if he remembers Me in an assembly, I will remember him in a better assembly (i.e., in the ...
As long as the deceased died believing in Islam, it is permissible and recommended to make dua for them. Doing so is taught in the Quran and Sunnah:
والذين جاءوا من بعدهم يقولون ربنا اغفر لنا ولإخواننا الذين سبقونا بالإيمان
And those who came after them, saying, Our Lord, forgive us and our brothers who preceded us in faith
— Quran 59:10