I have not yet found a solid set of correlated markets for CL, neither have I seen anybody convincingly laying out some markets that have a correlations as strong as those you gave in your examples. Truth is, CL is imho not the best market for orderflow trading purely based on correlation/divergence. It moves somewhat independent, which can be a good thing, or a bad thing, depending on your point of view.

Having said that, there are some things to look at: CL is a commodity that strongly influences (and is influenced by) the standard commodity indices, e.g., Bloomberg commodity index. It is also perceived as a riskier commodity, so it correlates well with rotations into risk. Plus the obvious dollar correlation (crude is international, so others have to hedge). I use JPY and GC to gauge some of those influences. But this is really more of a feeler for market sentiment than a correlation on any given day.

Correlations to RBOB Gasoline, Heating Oil and ULSD do also exist, but CL seem to lead those (anybody with some data on this?). Also somewhat Natgas, as it is another energy sector commodity. And Brent obviously, but the stat arb HFTs take care of any significant divergence in that. I personally could never make much out of these correlations when trading CL itself.

Important in CL might also be the various spreads. The crack spread for instance helps to gauge when refiners might have to react, but this is not as important intraday imho. Apart from that, many people and especially institutional investors trade calendar spreads on crude, so your friend might want to start looking into these as well.

That's my 5c. Any other ideas? I would be interested as well