Super easy. Spin up an OpenVPN server, forwarding the right ports to your server. Now spin up an Apache server with the folder your file’s in as server root. Send the client config for your VPN to your friend, along with the local address of your HTTP server. Now they can install the OpenVPN client on their PC and download the file from your HTTP server. Once you’re done, tear down all your servers, and don’t forget to unforward the ports. Couldn’t be easier.

/s