As the lounge in CMB is a UL lounge (not a contract lounge), under Oneworld lounge access policies you should have access for you and a guest to the Serendiva Lounge. As a OW Emerald you should have access to the Serendib lounge but ymmv as to whether the lounge staff will allow your guest in that lounge. You can always ask, I’m not sure you should be embarrassed (or reticent) about asking at the desk as you and your guest should still be admitted to one of the lounges.

As a business class passenger you personally have access to the Serendib Lounge but you do not have guesting privileges to this lounge as a business class passenger.
https://www.oneworld.com/airport-lounges?location=CMB
You might want to look at the other Asian airlines forum for other experiences. Or possibly in the QR forum as QR has been operating fairly consistently to CMB (although they usually contract with another lounge).