What should we do if a deceased person was buried without a funeral prayer?
If a deceased person was buried without his funeral prayer being offered, it is obligatory on others to offer the funeral prayer at his grave once. (So long as it is not regarded that the body may have started to decay).
And Allah ﷻ knows the best
Imam Ahmad Rida Khan رحمة الله تعالى عليه
Translation by Dr Musharraf Hussain Al-Azhari Translator of Majestic Quran, www.majesticquran.co.uk