Without an alternate reality where he had not died in that air crash it is impossible to prove he would have kept them up. His scoring record (roughly 1 in 3) for Nantes before he died even if he had maintained that record in the PL would have meant 6 perhaps 7 goals. Nowhere near what would have been needed to keep them up especially when you consider the number of chances they created on a per game basis and the number of expected goals vs actual goals scored.
Sadly relying on stats such as xG would be the closest way you could "prove" if he would have kept them up or not, and if you looked at his chance conversion rate and big chances Cardiff created and their expected goals it wouldn't have been anywhere near sufficient.
There is a reason they have lost every stage of this legal farce and the only injustice is that they have been given leave to keep appealing but after this there is no where else to go.
