Shutdown Computer in network via Macbook

Good luck! if something goes wrong, you might need a DeLorean to "undo"

It is easily possible with the correct knowledge of the computer in question and the network, it can even be done from anywhere in the world if your system and your target system are both on the internet! But before you get excited, there will be nothing but technical hurdles in your way and then legalities to be concerned over breaking if you are successful in your efforts. Most likely if you are at a public access point for internet there will be a security precaution known as isolation mode enabled to prevent exactly what you are hoping to do. Good luck finding a easy way around this without simply hacking the router at the coffee shop so that you have administrator control over it, and that would be very obvious to the owner if they setup and secured things like a good public wifi provider usually will. That aside, even if you do not have something preventing network access to your friends computers, they probably aren't just anonymously accessible. You will need not just a name and password to your friends computer, but one that has remote access enabled. And depending on the specific operating system, the ability to shut down the machine from a remote location might have to be explicitly set to be allowed and furthermore sometimes must additionally even named (Named - meaning you might have to know your computers IP address while on that public network and have your computers IP specifically programed into your friends computer as being a remote computer that has permission to shut down that friends computer). Also of coarse, if your friends have a security suite, a rule would need to be added to configure all of this to be allowed through their firewall. Now, even if you manage a solution to all that, are you sure your friends will take this jokingly? If not, it would be worse than being red handed, more like caught swimming in the paint bucket. If something backfires and someone wants to get back at you, you will have proven by all the steps necessary that you most certainly had an awareness of what you were doing, and that you continued anyway which shows malicious intent or some other legal mumbo jumbo I'm sure you can look into if ever necessary...