Some airlines do enforce it. BA definitely does (and even enforces not being able to activate a BAEC account until you take a qualifying flight, so you cannot credit discounted fares retroactively). Any of the plans could start to enforce the 4 flight requirement at any time. The only one that has never enforced it to date is AA, all of the other airlines have enforced it from time to time, and BA systematically and rigorously enforces it.

To answer the original question, without a doubt CX would be the best plan under their circumstances. Their chances of an op-up is quite good if they reach any CX status (and they should with 3 round trips on CX). They cannot qualifiy for the AA Plat challenge unless they are buying USD 3K+ roundtirps on CX (instead of the normal USD 1K discounted fares). In fact they would be better off buying BA WT+ fares if trying to do the AA challenge (1 trip would earn status for up to 2 years; their other trips could then be cheap fares on CX with lounge access and some op-up potential).