The Bung language is a nearly extinct language of Cameroon spoken by 3 people (in 1995) at the village of Boung on the Adamawa Plateau. A wordlist collected for it shows its strongest resemblance to be with the Ndung dialect of Kwanja, although that may simply be because this has become the village's dominant language. It also has words in common with Tep, Somyev, and Vute, while a number of words' origins remain unclear (possibly Adamawan.) For lack of data, it is not definitively classified.
Bruce Connell, 1997: Moribund Languages of the Nigeria-Cameroon Borderland (http://lucy.ukc.ac.uk/dz/connell/Mori/Moribundlngs.html)
Bibliography
Connell, B. (1995). "Dying Languages and the Complexity of the Mambiloid Group". Paper presented at the 25th Colloquium on African Languages and Linguistics, Leiden.
Bung has three people who remember it; the one who apparently knows it best is not a native speaker, though he learned it relatively young.
Certainly there are documented cases of language shift involving dissimilar languages, however it is not known whether, or to what extent, the degree of similarity facilitates or hinders the process of shift.
Declining population appeared to be a major factor in the death of languages in the area; causes for the fall in population may be linked directly or indirectly to either or both of the Fulani jihad and the Chamba incursions in the region, upheavals which took place during the 1800s.