Yes it will work hays47.
The Side Imaging transducer that your 899 unit uses has the same 200/83kHz elements inside of it that our standard 200/83kHz DualBeam transducers do and this is what the 957c normally runs on.
Although he will not be able to see Side Imaging sonar per se… have him try changing the Transducer Select menu setting to the “Quad Beam” setting and see what you guys think about it.