Permissibility of attending a nonmuslim funeral service for a deceased muslim

Assalamualaikum, I would be grateful if anybody in this forum could please provide evidence based on quran and sunnah as to whether it is permissible to attend a nonmuslim funeral service for **a deceased person who was a muslim**? (If the sources could be cited/quoted that would also be good so that we can disseminate the basis of your judgement to other family members.) The context is that my granduncle reverted to islam when he married my grandaunt, a practising muslim, about 50 years ago. Recently he also reaffirmed his shahadah. He resides in a nonmuslim country. We, the family, therefore believe he is muslim. However his will does not state anything about funeral arrangements. Some members of his nonmuslim (christian) family have been named as executors, and have allowed us to come to their **Christian funeral service** for him at the church. This is the basis of the question above. If we were to **attend** and sit but **not participate (sing read etc) in any religious rites** (therefore coming only as representatives of my grandaunt to her husband's funeral, and with a view that this would encourage a favourable view of Islam inshaallah among the nonmuslim attendees), would this be permissible in islam? I am very grateful and thank you for your consideration of this question. The family would greatly appreciate a response as soon as possible as the funeral ceremony might be happening in the next few days. Jazakumullah. Wassalam.