First, this is not the way multiboxing is normally done. Instead, once the characters are loaded, the "subordinate" characters are set to auto-follow the "primary" character. Then, using macros, specialized keybindings, etc., actions on the subordinate characters are triggered by the user's actions on the primary.
Specifically, there are a couple of technical problems with the approach your friend wants to follow:
- In order to reduce interference - imagine the confusion if 3 people in close proximity were using the same model mouse! - each mouse and receiver are coordinated so that the receiver only picks up signals from that mouse. You may be able to override this by registering the mouse with each of the receivers, though your mileage may vary. More likely, registering it with receiver #2 would break its registration with receiver #1.
- Even if you managed to get them all registered, you would also suffer from mouse pointer alignment problems. Basically, just because your mouse cursor was in a "safe" location on one PC doesn't mean that it is safe on the others. You could trigger all kinds of unintended behavior. You could reduce this significantly by hiding the UI (Shift-Z) on the subordinate characters. However, you could still suffer from targeting problems, for example, if a click event happened to target a mob that you didn't intend.
- Multiboxing often is done on one PC in order to reduce the hardware expense. If you turn down all UI, effects, etc. on the non-primary instances, the client itself won't strain the PC too much. One mid-to-high-range gaming PC is much cheaper than 5, even if each of the 5 isn't as powerful.