Mother-in-law needs parents marriage certificate to get pass

[Shevcraig]

Hoping someone can help. My mother-in-law, living in South africa, is applying for a british passport. Her father was a British citizen. She needs her parents Marriage certificate but they were married in Kenya. Kenya is going through a civil war and it is impossible to get hold of anyone. My mother-in-laws sister (same Parents) applied for her british passport while she was living and working in Saudi Arabia and recieved it no problem. She wasn't asked to produce the parents marriage certificate.
Has anyone encounted similar problems or can anyone advise on how to solve problem.
Thanks