It's my understanding you can only access different Oneworld lounges when you have the appropriate tier status with a Oneworld carries frequent flyer scheme. IE Emerald or Sapphire. If you don't, and are travelling in a premium cabin, then you can only use the lounge offered by the operating carrier, as Oneworld rules don't apply. At least that's how I understood it. The Oneworld website would be the best place to get the definitive answer on this.

Hope your Mum still enjoyed her trip, and the Malaysian Golden Lounge in Melbourne.

Cheers