Question: A person testifies that there is no god but Allaah and that Muhammad is the Messenger of Allaah, and offers Salaah (prayer) on time, however, he invokes (calls upon) other than Allaah. When that person dies, is it permissible to attend his funeral or not?
Answer: Asking Du’aa is a form of worship. Thus, directing any of it to anyone or anything other than Allaah is a major form of Shirk (associating others with Allaah in His Divinity or worship) which takes one out of the fold of Islaam. Allaah the Exalted says (what means)… read more here.