OK tips:
If possible take both lenses, but anticipate being limited to the 50 prime for shear speed. Our eyes are really good at dealing with low light so expect to be surprised at just how slow you'll have to go.
I don't know if the 7D has a similar option, but in this kind of situation I like to set my camera to what it calls TAv mode. That means you set the aperture and shutter speed, the camera sets the ISO. You may not have the specific mode but might be able to get the same effect by setting M mode and auto ISO. When set to this mode my camera displays the ISO in the viewfinder so I can keep an eye on what it's doing and adjust shutter speed/aperture as necessary to keep the ISO within acceptable limits.
Don't be afraid of high ISO. Particularly with your 7D, you've got options up to 12800 so as a guess I'd expect up to 3200 to be useable with some noise reduction in PP. Hell I've made very acceptable prints at smaller sizes from my K10D at ISO 1600 (its maximum) so with the more modern sensor in the 7D and a bit of cleverness you should be fine.
If it's apparent you're only going to be able to use the 50mm prime (quite likely unless the 17-85 is f/2.
stand back as best you can and make use of the image quality advantage of the nifty 50 to crop in later.
If you can't get a shot without motion blur, don't try, use it to convey the passion or speed of the music. ie if she's playing a guitar a bit of blur on the strumming hand can be really effective. Takes a bit of timing and a lot of luck of course.
In my experience, if you want a portrait orientation shot, if possible still shoot landscape and crop later. I always find the camera easier to hold steady in landscape orientation. Unless of course you've got a battery grip with auxiliary controls in which case you can hold the camera whichever way round you like.
I'll bet whatever you get is better than anything she's had before. This kind of situation is where a good DSLR with a fast lens really pulls ahead of the compact crowd by sheer light gathering power and high ISO performance.