What is the maximum amount of memory Adobe Premiere Pro can efficiently utilize?
That entirely depends on what kind of footage he work with and how long his average cuts are. If we are talking hour long movies a bigger amount of RAM can be worth it.
Though 128GB would be completely useless if he only uses Premiere it just doesn't use that much RAM.
We built a high-end workstation once for huge 3D renderings that has 64GB RAM, the only times we utilize that fully is when working with gigantic 3D scenes in Cinema 4D or 3Ds Max. After Effects barley goes beyond 20GB unless you need an hour long RAM previews for some reason and Premiere is usually around 5-7GB even when cutting 60min plus movies.
So if hes only using Premiere, 128GB would be a complete waste of money. I would not recommend anything more than 32GB on a video editing only workstation.

When using Premiere a high-end GPU is a lot more worth performance wise, Premiere can render in realtime with a powerfull GPU making having a ton of system RAM a bit useless.

I can recommend using gaming cards instead of workstation cards, the only difference is support and drivers, the gaming cards are much more powerful and a lot cheaper. I have never experienced any stability differences in Adobe software with a Workstation card compared to a gaming card. A good mixture of gaming GPU and workstation GPU is the GeForce GTX Titan, its meant for CUDA development and other CUDA usage.

Alternatively in the latest Premiere version there is also full OpenCL support, so a high-end AMD card would also be a good choice.
F.e. The R9 290X or if he wants a workstation GPU for some reason the FirePro W9100 is currently the high-end model, though a waste of money in my opinion. You do not need 16GB Vram with Premiere. I would probably go for the W9000 or W8000.

Edit: As AJ Henderson mentioned, footage gets almost never fully cached in RAM so a fast SSD or maybe an SSD array will be much more beneficial than more RAM aswell.
Going for fast disk I/O and a good GPU is a good combination nowadays. Having a high-end multi core CPU and lots of RAM is getting less and less important in the graphics world.