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.

get a flash

if he's worried about the noise of the shutter causing a disturbance, I'm guessing he doesn't want to use flash.

I'm also still being surprised by the MP on the 7D, in terms of how much I can crop out and still have a usable image with whats left, compared to the 40D which had near half as much resolution.

I'll bet, with a good prime lens on I'm constantly amazed at how much cropping I can get away with even with my realtively meagre 10MP

This

was the central 2MP shot with a 90mm macro prime wide open at f/2.8. Obviously no where near as low light as you'll be in, but it is quite amazing what you can get away with in terms of cropping provided you don't want to print at poster size.

She knows I dont do people...and its the first gig she's done in front of friends etc, so its all practice for both of us.

I was planning on doing the others too, doesn't hurt

so what kind of band is she in zpyder ?

If you're going to shoot with the prime, position is everything so use the previous bits to work out the best angles / positions for your lenses + not going too smashed about etc. Then, when your girlie is on stage, use your elbows to get back to that spot. 

She's doing solo acoustic stuff.

I've thought about lens renting, but I don't think this gig warrants it, not yet. Bit like learning to play a new instrument using a really expensive one, you won't get your moneys worth till you get some practice in. I might end up doing a colleagues wedding later in the year and I might suggest to them if they could pay for lens rental and thats it. She's only just starting out on the solo gigs and I'm only just starting out doing people so it should all be good.

No tripod, I'll just use high iso, should be ok at ISO 800 I think (or more if needed, it goes up to uber-noisy 13200 (i think thats it)) and noise reduction. I'll be shooting in raw so I'll have a bit of freedom, and there's tables and chairs to use as pseudo-tripods.

I'll start on the prime, as I'll be getting there with her, before the open mic night starts proper, it'll give me a chance to see what kind of range I have. It's quite a narrow and short venue so if I can hug the wall and have 6' of forward/back movement I think that'll do. If I can't get back enough to get some of the stage in I'll just have to switch to the 17-85mm. If necessary I'll sell the 17-85 and some other tat to get a better lens, the 17-85mm is F4 I think.