Adobe has indicated that there is no list of supported graphics cards at this time. Unless your customer has a Mac with a high-resolution retina screen, the likelihood of changing graphics cards speeding things up doesn't seem to be very great.
Does your customer's setup use 4K/5K monitors? If not, then don't expect significant performance increases with a GPU. Spending money on a new GPU may not gain much.
See this info from an Adobe engineer: GPU notes for Lightroom CC (2015) | Adobe Community
My customer is using a custom windows based machine. They are NOT using a 4k monitor, however they are driving a 2560x1600 monitor.
On my system with an an i7-860 cpu and Nvidia Quadro 600 graphics card there's no benefit from LR CC GPU support. In fact when browsing image files in the Develop module there is an additional `1.5 to 2 sec. delay before the image becomes fully sharp. I suspect my i7 860 Quad Core processor is actually faster than my Quadro 600 GPU for updating the image previews, so I have it disabled. In addition there have been numerous reports form people with AMD GPU based graphics cards due to driver incompatibility.
My best suggestion is to get the highest performance Nvidia graphics card the customer can afford. If the customer also uses PS and a 30 bit (10 bit/color) monitor you will need an Nvidia Quadro model card. AMD FIrePro supports 30 bit color, but as mentioned many users currently have driver compatibility issues with LR CC. PassMark is a good site for benchmark comparison:
If the card is strictly for speeding up LR CC I'd hold off for a while until the dust settles.