A Muslim was buried in a Christian graveyard, and no funeral prayer was offered for him. What should they do?

No File Exists
Check below answers in case you are looking for other related questions:
She refuses to wash deceased women any more lest she becomes desensitized
Medical tests on the body of a stillborn infant

No File Exists
Related Answers:
A dhimmi woman who dies when she is pregnant with a Muslim child
If there is anything on the body of the deceased that will prevent water from reaching the skin, should it be removed?
The forty-day anniversary is a Pharaonic custom
She refuses to wash deceased women any more lest she becomes desensitized
Medical tests on the body of a stillborn infant

Recommended answers for you:
Visiting the sick – some etiquettes
She did not eat suhoor and did not wake up for Fajr. Should she carry on fasting or break her fast?
The proper way to make du’aa’ during prayer
Is it better to fast on Mondays and Thursdays or on three days of each month?
Ruling on shortening prayers whilst travelling