I know a type of plastic clip available over here(parketdeck I think they’re called) where the installer uses Sikaflex above and beneath the plastic clip he says to stop the boards rattling in time. I don’t know however if that’s his own preference or if its a recommendation from the clip manufacturer.
Personally I don’t use glue in such a way but always have a bottle of waterproof polyurethane to hand for where it is needed, like plugs and railings etc. Applying glue beneath each board seems a little ‘extra’ to me and I’d rather have faith in the clip and its screw to do their job without a backup.
You’re client needs to consider the fact that for invisible installation he’ll want thinner boards and the extra costs that this entails paying for the clips and your extra work in covering the same deck area with those thinner boards.
good luck with the client