Since I was a member of that close-vote tribunal, I'll offer up my own thoughts. I voted to close as off-topic because:
I have some friends that are non technical that live overseas.
This tweaked my radar right out of the gate. ServerFault's FAQ restricts the site to IT professionals who are working on projects that are within a professional environment or for professional purposes. That doesn't necessarily mean "Work in exchange for cash" either. It has a finer meaning of "not tied together with chewing gum and bailing wire." However, if you can edit out any reference to a home-use scenario, and the scenario can can still stand on its own as potentially professional, we'll probably answer it.
We have something of a "Don't Ask Don't Tell" policy about home stuff. However, beware that home scenarios often come with odd caveats and restrictions that will inevitably require someone to either drop the subject or confess that it's a hackneyed home-use situation. Witness this question that smelled funny to begin with, but then in the comments was eventually found to be a non-professional that was in need of educating (which is fine, it's just not exactly what we're here for).
They need to be able to download a very large file (10+ gigs) but
their internet connection is spotty at best.
This is the core of your scenario, and could probably be asked without reference to home-use or non-technical friends. I'll see what I can do about editing it to be on-topic.
FYI, you might try stating your scenario and then asking "How can I offer file transfer resume support?" rather than scoping people to SFTP. Unless, of course, you have a technical reason to go with SFTP. Just an idea.
As they are non technical, they have no familiarity with rsync
This isn't necessarily the kiss of death for a question. Many of us deal with non technical users, clients and customers for whom we need to set up complex systems that they will interact with on a push-button basis. However...
plus I don't know how to get rsync working on Windows.
This one grated on me ever so slightly. It pretty much sealed my close vote. In my opinion, the general consensus of ServerFault is: Not knowing is forgivable. Not learning is unforgivable. You don't know how to get it set up on Windows? Learn (as long as that's the best option in your scenario).
My question is: does Linux's SFTP via SSH support file transfer
resume? I have heard that both the client and server need to support
file resume for it to work. I'm guessing it will, because rsync over
ssh does, and that's just on the client side.
And the final nails have been driven into the question's coffin. It's one thing to ask for guidance, but this is bordering on a RTFineM question. It would be better to show that you've at least put some research effort into things by saying "I've read the man pages but can't discern if resume is supported." Even better: "I've tried to get it to work by doing thus-and-such, but the results were this-and-that. What could I be doing wrong and how can I troubleshoot this?"
Never fear! I'll go about trying to edit your question to into an on-topic state and see where it goes. EDIT: And I took your question and heavily edited it. See what you think.