Praise be to Allah.
We put this question to Shaykh ‘Abd-Allah ibn Jibreen, who replied:
If when he died he was persisting in calling on the dead for help, it is not permissible to follow his funeral procession. If nobody discussed this issue with him, we should look to see whether he was in a land there were any pure monotheists who propagated the message of Tahweed and if there were any ways in which he could have learned about Tawheed. It is most likely that proof was established against him, so we should not follow his funeral procession. Otherwise we may do so.
And Allah knows best.