Help to ID a mysterious fruit for a friend?

[Richard]

Kei Apple.

Kei Apple

This plant is “apple” in name only. The species is Dovyalis caffra from the willow family in southern Africa. The word “Kei” is likely from its native presence in the Kei River Valley. The plant is dioecious and thus to start it from seed you must wait until the seedlings bloom and then select male and female individuals. It has large hardwood thorns and a long history of use as a protective barricade from large wild animals. Consequently, if you wish to harvest the fruit and retain flesh on you…