please take the time and make your friend some bullet cartridges for his dschungel gun. I dont now which brand this single shot is and how strong they are, believe CBC has made such guns.
Magtech 28 ga brass filled with elephant black powder and a patched hardened lead round ball may help to bring the buffalo down with 4-5 shots.
I suspect that you dont see a jaguar regularly in dayligth. do they kill water buffalos?