The keyboard player is not defining what problem he's trying to solve by suggesting that a totally different right hand technique is used. He could say "I think it needs more definition" or "More treble" or "don't do any right hand muting. Its like when a website designer shows a mockup and the client says something like "make the buttons blue" - why do they think they need the buttons blue? Is it to make them stand out more? Make them fit in with the colour scheme of a parent company's website? He's a Chelsea supporter? Or fan of the boyband ''blue? By communicating what result he hopes to achieve by suggesting a pick is used, there could be alternative and better solutions. For example if the problem is that the bass is not cutting through the mush, a better solution might be for the guitar &/or keys to adjust their sounds make a bit of space in the midrange for the bass's mids.