Even if swaps could work on an OEM rig like Mac's, the i7 975 is an LGA 1366 and a desktop processor whereas the C2D is a LGA 775 and a notebook (in this case) processor.. So, its an awesome NO CHANCE on your swap plans..
even if you could put it in, i wouldnt really recomend it - the stock cooler that comes with the cpu is like a "minimum heatsink". If the mac has a heatsink, i doubt it would be sufficient and i srsly doubt you can fit the stock cooler in the mac.
If the mac is used for powerful tools, it would take alot out of the cpu and overheat like hell (it might burn or just break).
Sell the i7 and MacBook Pro and buy a new system for your wife. If she wants a Mac, the new MacBook Pros with the i5 processor are pretty sweet. If she is willing to go with a "PC", your options are virtually limitless.
Now, let me ask you this. When you say the MacBook Pro is slow, what do you mean? I have a similar MacBook Pro 15" that is pretty capable and is very responsive. What are you doing that is running so slowly?
Why would you want a 975 in a notebook anyways? O.o
Now why having that cpu in general, i couldn't say...
Although why would you have a desktop cpu in a laptop form factor?
well if you're a power user/enthusiast that like the power of desktops but going to be in moving around frequently/ need that kind of power in a remote location.... A desktop nor a normal laptop not going to cut it. That were the specialty desktop replacement laptops come in.