I don't know the rules are officially, but from my experience in the same situation BUT at MEL - it was MH operated but marketed by TK - I was allowed into the Qantas First lounge (based on my Emerald status) - the flight did NOT credit into my OW program, and it was hell getting TK to credit it too.

This is why since than I always try to avoid these cross-alliance codeshare as it's just a nightmare to know what you get or can't.

In this case, I think there could be a chance that the pax will be allowed into the MH lounge since MH is operating the flight (normally on the BP, it's the main operating airline showing up, while I for sure know with a TK operated flight and marketed by MH, one is not allowed in, as I've checked before at KUL just recently).

Cheers!