Thank you @Uli for your suggestion. I was entertaining the idea that it might have been a juvenile Agapanthus praecox, which are commonly grown in surburban gardens. I'll admit that the plant in the photos more closely resembles one of the Chlorophytum spp. (I chanced upon a juvenile Agapanthus in the field the day before, and the appearance is quite different.) Flora of Victoria (the State Flora) acknowleges the presence of C. comosum: "Widely cultivated as a house plant, with variegated forms common in cultivation. Both the variegated 'Variegatum' and non-variegated form are naturalised around Melbourne." If my colleague does keep the plant, we can identify it when it flowers.