Once again you're splitting hairs. Do you think granny will care that it's officially called a virus or a trojan? Which by most definitions of virus a trojan is just a subset of a virus. Like the Cold and Flu are both a type of virus.
But let's consult Mariam Webster: click here
Now read item #4.
Or you can trust me and read it here:
4 : a computer program usually hidden within another seemingly innocuous program that produces copies of itself and inserts them into other programs and that usually performs a malicious action (as destroying data).
So to make this a "virus" all you have to do is turn granny's computer into the host attacking other Macs on her network. This way you won't have to leave your Mac on listening for Software Updates. Instead you use Granny's computer to do it. Just install the client software on her computer and you officially have a virus.
No, there is a distinct difference... A Trojan relies on psychology to attack a computer, a virus replicates and installs itself on the target computers. Granny's machine will not install this on other computers without THAT user's intervention. There is no vector.
Without the "victim software package" the client computer will not connect to Granny's computer. It will ignore it and connect correctly to Apple. Just because Granny's compromised computer is on the same network does not cause other Mac's on the network to spontaneously also become infected. The only computer that will download malicious software will be Granny's. Trojan, not virus.