The proper way in Lua to use arguments in a function created using loadstring is vararg expression, written as three dots.
local argument1, argument2, argument3 = ...
In your case, the function body would look like this:
local player = ...
return getPlayerName(player) == 'owner'
An example with loadstring:
local func_str = "local player = ... return getPlayerName(player) == 'owner'"
local loadstringed = loadstring(func_str)
local testPlayer = getPlayerFromName("player1")
local success, info = pcall(loadstringed, testPlayer)