You will of course need a Linux OS running (I advise against cross-compilations). For that I suggest you get VirtualBox or any other VirtualMachine that can run a Linux distro. Then install the distro of your choice (maybe make it match your friend ones) and install all the needed development tools (GCC, etc.).
After everything is up and running, you can get your Windows code and try to compile it. If you've written good platform-in-depended code, things could even compile. Otherwise you'll have to go and add some
#ifdef WIN32 preprocessor directives or abstract it as nicely as SFML does with its code and implement the Linux part.
It will be time consuming for sure!
Btw. for Linux you can most of the time also bundle your Windows application with WINE (WINE Is Not an Emulator) and it should run on Linux.