MIDI or Musical Instrument Digital Interface is a control method. It deals with control information and not the actual signal itself. MIDI is not normally bi-directional. A MIDI jack will normally be an input or an output and not both. There is often also a "thru" for routing to multiple devices. MIDI control will control note information, patch changes, etc. I'm not exactly sure by your question what your intent is. If you could more specifically explain what it is you're after, it might be easier to give you answers. If she just wants to practice with drums, the cheapest/easiest option might be to run the computer as a drum machine using it's own speakers and the guitar rig separately.