So, the main question is, can one create expertise through using proficiency and another proficiency?
Let me try and explain, A level 9 mind flayer savant player character would regularly roll +13 to perception rolls due to a skill that allowed skills to be int based instead of wis or cha (ignore the stat switching) and so the equation comes out like this: 5(int) + 4 (proficiency) +4 (race proficiency) = +13 to perception rolls and a passive perception of 23.
He claimed it was kosher for race proficiency and trained proficiency to stack, saying it was in the PHB. I can't find anything like that and the internet is stuck on teaching the difference between base proficiency vs expertise.
So in short: Is there a way to stack proficiency (from any source) or is that him misunderstanding (or just wrong)?