The absolute simplest way would be to set up a call with your friend and have him hold the microphone (and key the radio) next to the speaker when you want to talk, and hold the radio speaker next to the phone microphone when you want to listen. Works, but terrible phone-quality audio.
The next simplest way would be to have your friend install Skype and set up a sound card interface between the PC and radio. Have the radio pre-tuned to the net and in VOX mode. When you speak it will key up the radio. A good quality microphone and headset designed for VOiP should give you decent audio.
If you want to control the radio, your friend will need to install rig control software, a CAT interface and some sort of remote PC control software. You'll need to connect using your PC and similar software (lots of options, many free, available). You'll continue to use Skype for audio.
If your friend has a complicated setup, such as multiple antennas, you'll need some way to switch them and it gets a lot more complicated. Modulated CW might be possible with a code oscillator, but might be more trouble than it's worth trying to keep a clean signal. Someone else might have some experience with remote control CW, I haven't tried it.