How can I send a message to an iPhone from a Windows machine through WiFi?
It's (virtually) impossible.
It would be a very big security flaw if anyone on a network could just run some code and make some message appear on my phone, unsolicited, don't you think? If that were possible, for instance, you could send prank/spam/virus messages to anyone on any somewhat public network you've ever been on. Think airports, starbucks, etc.
And the only reason I added the "virtually" there is because, technically, with a lot of resources, skills, money and some zero-day vulnerabilities, it could be possible, but that's way beyond the realm of real life in any practical scenario.
But a somewhat obvious solution could be: just change the password, she will come by again to ask about the change. Get your tape.
Or ask the neighbors as well.
I would like to qualify Vinicius's answer. It is possible to send messages to other computers on your network. You may be out of luck in this case, but it is otherwise totally possible to do this.
This was (and is) possible with the
net send command.
net send has since been superceded by
msg. On a domain, you can simply type
msg username message at the Command Prompt and your message will be sent to that user. You can also target a computer instead of a username.
I have heard
msg is restricted to certain versions of Windows (it works for me, but I'm on a domain right now), so you may have better luck with the following Microsoft provided tool: https://gallery.technet.microsoft.com/scriptcenter/A-net-messenger-script-for-810f8331
netmessenger-eng.vbs. The one beneath that is Portuguese.
This particular tool is setup to send to the entire network, since I am on a domain at present, as you can see below:
You may have to run enablemsg.vbs if it does not work for you at first.
You can try this tool, but I highly doubt it would work with an iPhone. If she had a computer, you might have some more luck.