Is it permissible for a Sunni Muslim to attend the funeral of an old friend who is a heretic?

It is permissible to attend the burial of an innovator, and one should denounce any innovations present in their funeral. If this denouncement is not accepted, then some scholars say one should not follow the funeral procession, while others say one may follow it while denouncing the innovation in one's heart. One should not abandon following a funeral procession due to an innovation committed by someone else.