Without any in-built attenuation anything you do to the internal microphone will probably affect the frequency response. An external mic with high SPL handling would be the best option. Pointing an external mic away from the stage might also help. Usually in situations like this you need good monitoring, a pair of DT100's have fairly good insulation properties.

If your mate is in the band and you're doing it for them. Get the sound guy to provide you with a PA feed, admittedly it'll be mixed for the PA but it will sound miles better than a microphone.