havent done this in a while,but I concur, he should take the ratio of SPY/AAPL and start there..You can beta adjust it,vol adjust..I would check the ratio of the historical rolling implied vol as well as the ratio of SPY to AAPL..Scaaawwwwy trade
Its not the IV diverging,its the price ratio going bonkers(AAPL outperformance).You are correct that depending on IV levels,he MAY be able to short spreads in AAPL and go naked long SPY vol.
In colin bennet's book, I remember a section where he compared/backtested the different methods and beta adjusting was the most accurate. Of course then there is the problem to decide on which timeframe to calculate beta
also, going back to the original topic, you should watch the rolling correlation of the two and try shifting it back n times. If the correlation relation is not holding well over different periods, then this is a red flag
AAPL much more correlated to QQQ. A call spread on the pair? Why not just long/short the pair instead with an appropriate weighting?