To my knowledge, Mac Pros are workstations and workstations cater to businesses. Because of this, the tower was built with an IT support staff in mind. In other words, the computer is very modifiable. Your main limiting factor is going to be the power supply and the connections on the motherboard. As long as you don't put a graphics card in that will exceed the psu, you should be fine. Without seeing a unit personally, I would imagine that you can upgrade RAM, hdd/ssd, optical drives, graphics cards (to an extent) adding soundcards and other pci cards and possibly the heatsink on the processor.
Where I work, we are given HP workstations that feature xeon processors, cuda graphics, etc. There are no compromises made when it comes to business and spending a couple extra grand can mean thousands saved in time over the life of the computer. If this computer is for business, I would go the route your gf feels most comfortable with regardless the cost (within reason of course). But when choosing an Apple machine, I would wait to see what apple will be unveiling with the new Mac Pro tower (coming out later this year). I cannot speak to how serviceable the new model will be though.
Right now is not a great time to buy a Mac Pro. The unit has not been refreshed significantly in years, and the rumors are something new is coming in the next few months.
A couple of other notes, there are only a few video cards that work in the Mac Pro, and none of them are that exciting.
I don't believe you can modify the processor heat sink. The stock cooling configuration, however, is thought out very well.
On the flipside, some believe that the new release will not be a Mac Pro at all, but rather a thunderbolt box that allows expansion of an iMac into something with Mac Pro capabilities. This is based on parsing Tim Cook's comment wherein he does not say a new Mac Pro is coming but rather that something that people who are waiting for a Mac Pro will like. This would be consistent with Apple's movement towards eliminating optical drives and favoring SSDs.