Just my opinion: When your friend is currently using a TG for macro, MFT is certainly a worthy upgrade that will lead to substantial improvement in IQ. The practical value of MFT for macro is very good with the Zuiko 60mm and, for less clear waters, the Pana 45mm, both sometimes used with CMC-1 (seldom needed as 1:1, as provided by the lenses alone, already allows a subject that is 17mm long to fill the photo). In addition a 90mm IS macro, that allows even 1:2 magnification (8.5mm will fill the photo, even without closeup diopter)), will be announced soon by OM systems. This lens may well be very useful for macro UW, but we will see...
=> I think a OM-1 camera in Nauticam housing plus the macro lenses above are the best choice with regard to usability and give very good IQ.
=> In case your friend is out for the very best IQ available, a high resolution FF camera (e.g. Sony A7RV, but also DSLR) may be better choice, but the rich choice of lenses does not exist at present (and maybe never will). To fill the frame with a 8.5mm subject e.g., one needs an additional strong diopter to the macrolens that will provide a 1:4 magnification)...
Wolfgang