You are using a shutter speed faster than 1/250s.
The D7000's "maximum sync speed" (aka x-sync speed) is 1/250s. Most dSLRs these days have focal-plane shutters in them, where the shutter speed is determined by the size of the gap between the first and second shutter curtains as they sweep across the sensor. The smaller the gap the faster the shutter speed. At 1/250s, the gap is still larger than the sensor. When you go faster than that, the slit is smaller than the sensor.
A flash burst is typically much much faster than your shutter speed, so when you go over the x-sync speed, one or both of the curtains will be covering part of the sensor, and you'll get a black bar at the top and/or bottom of the frame because the light from the flash is shaded from the sensor.
You can go faster than 1/250s if you use "high-speed sync" (aka FP or focal plane flash). The SB-600/700/800/900 and your D7000 in concert can do this. Basically, the camera will tell the flash to pulse and act like a continuous light source for the duration of the exposure. The main drawbacks to this are that it requires both a camera and a flash that can do Nikon's version of high-speed sync communication; and you'll have a power loss from all that pulsing—typically about two stops. Neil van Niekerk has a great article about high speed sync.
A Canon flash can only do Canon's version of HSS communication. :D If you look at the pin-and-contact arrangement on your Nikon camera and the Canon flash's hotshoe, you'll see that the pins (except for the one in the center of the foot's "square") are placed in completely different locations from the contacts and there won't be any signalling other than with the center pin "sync" signal—which tells the flash when to fire so that the flash burst happens during the exposure.
Don't worry about people saying you may damage your camera from voltages or crosstalk or whatever. Both Nikon and Canon modern dSLR-compatible flashes use voltages under 10V, and both hotshoes have a limit of 250V. And as long as the flash is seated properly, there won't be any contact of the TTL signals. You can also always tape off those non-center pins if you're really paranoid. I've used a Nikon SB‑26 on my Canon 50D without any issues.
One other trivial note: if the Canon flash is a 580EXII or 600EX-RT, while you can't use it on a Nikon body with TTL, you can still get automation of the flash's power by using the Ext. A mode. This is like the old autothyristor modes you used to find on flashes. Your friend may not be aware of this feature, as it's buried down in Custom Function 05 (set it to 3), and most Canon shooters use eTTL instead. I shoot micro four-thirds as well, and have used Ext. A instead of TTL on my Panasonic camera. Autothyristors use external sensors on the flash to cut off the flash power, rather than flash/camera-body communication and metering a preflash like TTL schemes do.
