To the metal part of the casing so that you, the memory and the computer are all at the same potential difference.
In the old days you were always told to leave the computer connected to the mains so that you could be certain that you were actually grounded, but I think the health and safety police decided that was a bad idea, better to zap a computer than zap a human, though given it's your girlfriend's Mac not yours, you may decide different🙄
But yes, if there is a conductive component in a computer, say the metal power prongs on the back or battery prongs, that's the best place to discharge.
Actually, there's a fair chance that the power pins are made of aluminium too, as are the power cables in the building and almost certainly the HT power cables that carry power from the generating station to the local substation.
Aluminium is a very good conductor, it's only slightly less conductive than gold, which is used in most of the other components of the computer.
The only widely used things that are more conductive are silver, copper and gold (in decending order). Aluminium is used because it is lightweight, relatively environmentally inert (compared to silver and copper) and much cheaper than the others.