During the ceremony, definitely use the GoPro as a superwide safety shot. This will give you something to cut to during the edit when you're adjusting focus, changing angles or the record limit stops on your DSLR's. Depending on the venue, placing it in a balcony would be ideal, but it can also be placed on a mini tripod on the floor facing the altar/stage. Hero 3's have terrible battery life, so don't hit record on it too early (10 minutes early will suffice).
I would put the 6D + 17-40 on a tripod (or monopod if it's a version with balance feet and you're confident in being stable) in the front, facing down the aisle during the processional to capture the wedding party and bride entrance. Obviously don't block the aisle, and be mindful of the guests. As for focus, if you're not experienced I would NOT try to manually follow your subjects as they walk towards you. Instead, place the focus towards the middle of the aisle (or closer) and allow your subjects to walk into the shot. Once they're out of focus in the edit, cut to the GoPro or 70D, then cut back to the 6D for your next subject.
The 70D should be placed in the BACK of the venue on a tripod, equipped with your 24-105 (the crop sensor will turn that 105mm to 168mm). During the processional, this will be your groom shoot. Keep in mind it might all be ruined when the crowd stands up and blocks your shot, so you'll have to prepare for this. After the bride has been handed off, readjust the tripod to the center of the aisle (but not too close) for a nice, tight middle shot on the couple. As a professional, I never trust auto focus, but you may want to use it here if you're not confident with the LCD screen. Just make sure to keep an eye on it, and if it's constantly shifting, manually focus on the couple and leave it alone.
At this point your 6D will be shooting the crowd (aka: nothing important), so go up and readjust it to face the couple from the side. Pop on that 100mm (you could have it lying below the tripod) and get a tight shot of the bride or groom, whichever you are facing. If you had a third DSLR, it would mirror this one on the other side. Alternatively, you can just set this camera aside and only worry about your middle 70D shot, as you're probably not getting paid for this anyway, and the middle is the most important. However, if you're not going to mic the groom, then the 6D can be your slightly-less-crappy audio source. Even if you put an external mic on the camera, the audio is still going to be trash due to the terrible preamps, but I digress . . .
Or, you know, tell her to hire a freaking professional