Is It Ok To Dot and Dab Plasterboard to Concrete Lintel Above Windows

Hello All

I'm getting some plastering done around new windows

The is a 20mm gap between the top of the window and the concrete lintel, I wanted all the undersides of the lintels done in sand and cement but the plasterer said this would be too thick and heavy and would be best to stick plasterboard to the lintel and skim over it

Would this be OK or should I insist on sand and cement?

Not sure of the best way forward

Thanks