He will get audio; either a blank audio track (it is recorded audio but with no signal being sent to the interface, or possibly background noise).
It will not record any rendered material or his synth via the USB (which on his unit is MIDI only for realtime)

But if he has an interface with any inputs enabled in RB it will record whatever comes into that input as well as the intended MIDI.
If he decides to keep the audio it would be put on the next available track.

As noted previously there is an option to enable/disable this feature.
The original intent was if someone recorded the vocal track of a life time but accidentally recorded it on a MIDI track, RB would salvage the mistake.

Make your sound your own!
.. I do not work here, but the benefits are still awesome