All signs seem to point to me purchasing the 70-200mm 2.8L IS lens in order to handle action/low light situations.
My question is this: Is there another lens I should be considering that can handle hockey as well as portraits? My wife is a wedding planner, who is interested in possibly becoming a wedding photographer.
I've heard good things about the Sigma 50-150 f/2.8. Much lighter and easier to handhold than a 70-200 beast, much cheaper, and gives you the same coverage on an APS-C body. That said, it is only for crop frame cameras so if she ever upgrades to FF, you're out of luck. The Canon will also probably give you better quality wide open, but at double the price, it should.
If she is serious about being a wedding photog, then EF 70-200 f/2.8 IS (II) without question. The IS isn't going to do much good for hockey (players are moving to fast for it to be of use), but for low light ceremony shots where you can't use a flash, it is extraordinarily handy. Also, she is going to need at least two bodies for wedding work (no skimping here, at least two bodies is critical, not just for focal length coverage, but for backup, bodies go bad at the worst times), one of those is probably going to want to be either full frame or 1.3x frame (1D). You are going to want a stable of lenses that will work on all her cameras.
Now for the nerdy talk (skip all this if you like

):
Here are the MTF charts for both lenses. What you are looking here is the lenses ability to resolve fine detail (green lines for Sigma, fine black lines for Canon ) and contrast (red lines for Sigma, bold black lines for Canon). The contrast line will indicate how much "punch" and clarity a lens will deliver, the detail line will indicate how much fine detail (personally I find that while both are important, the contrast line is the more important of the two). If you are wondering what dotted vs. solid means, it is the direction of the test line on the chart. Lenses have a harder time resolving lines which run perpendicular to a line drawn from center of frame to edge of frame.
Y axis (height of chart) is contrast (1 would be perfect contrast), X axis is distance from center of frame, so the left side is center, the right side is the extreme diagonal corner of frame. One important thing to note, the sigma chart ends at 13.5mm on the X axis (because it is for cropped cameras) while the Canon chart extends to 21.6mm, just keep in mind when comparing the two charts to look at the same X value.
Sigma:
Canon:
So what you can see is that in fact on wide end, the Sigma has slightly better contrast while Canon has slightly better detail across frame.
On the long end the Canon really shines. Better contrast, and much more detail resolving power.