I'd say it really depends on what your friend hopes/needs to get out of a program. Even though both are OWE, they offer very different benefits, no?

I know more about MPO than BAEC, but as I understand it:

-- Seat guarantee is more generous for Y flyers in MPO than in BAEC. On CX, the OWS/OWE-equivalent tiers can guarantee into lower coach booking classes, while in BAEC it's only full-fare Y.

-- If he misses DM one year and ends up as a GO, he can still pre-reserve exit seats on CX, which I don't think is possible for BAEC's OWS equivalent level.

-- RDM tier bonuses don't exist in MPO but do in BAEC. Although with BAEC redemptions require huge surcharges (CX surcharges are merely big )

I think it matters a lot which carrier he's likely to fly the most (CX also offers more frequencies at different times on HKG-LHR-HKG). I personally would lean toward MPO because the tier benefits are quite helpful. But if redeemable miles factor into the equation at all and he's willing to fly mostly on the BA flights, BAEC could still be attractive even at the OWS-equivalent level.

To clarify: I know OP didn't start out asking for a relative comparison of the two programs. But it is useful to think about that if only because, if something like RDM tier bonuses is at all important, the friend could end up being better off as a OWS-equivalent in BAEC than as a DM in CX.