Scrim tape is for plastering over really.
I had a number of cracks in one of my bedrooms so i put some noggins up in the ceiling above some of the cracks then used plasterboard screws to screw into the ceiling either side of the wood, then put scrim tape over the cracks and skimmed the whole ceiling, it was probably overkill but no cracks have re appeared since and that was in 2015
. Is the ceiling on the ground floor or upstairs ?
Whilst reskimming the entire ceiling using scrim tape would remove all the cracks, and less likely to re-appear that is a bit drastic if hes using the room as reskimming with gypsum finishing plaster is not only a skill, its also bloody messy ... He would need to ideally remove as much furniture as possible .
His best option is to scribe out a 6mm v shape line where the cracks are, and then fill with Easifill 60, its less likely to shrink and will provide a decent finish if he applies with a 10" flat float trowel - if using a scraper then he can level it out best you can , then sand smooth - if he uses a trowel he can feather it in using water just before its going off to get as smooth and flat as possible to reduce sanding
The Scribed out line will give the easifill something to wedge into, and will prevent cracking hopefully, and should be less mess and should hold up
Caulk won't work as you can't really sand it, and it shows a visible line , its fine for cracks in corners, around skirtings and door frames etc - but in the middle of a ceiling or wall you will just see the line of caulk soon as you apply paint.
