Running mechanical on di2 frame

I'm contemplating purchasing a DI2 frame and running mechanical on it. The seller states:

"ELECTRIC OR MECHANICAL SHIFTING FOR MECHANICAL SHIFTING YOU WILL NEED A CABLE GUIDE UNDER THE BB."

Is this factual? If so how labor intensive and skilled is the job at hand? If it does work will shifting performance suffer?

 

Road bikes traditionally have the front shifter cable running down the down tube, changing direction at the BB shell and connect to a bottom-pull front shifter.
To keep the cable from wandering, and from eventually sawing into the BB shell, you need a BB cable guide. They look like this:https://www.bing.com/images/search?q=bottom+bracket+cable+guide&FORM=HDRSC2
Fitting one is neither rocket science or brain surgery.
If done about right, you should have a bike that shifts as well as a bike built for mechanical shifting from the start.

 

It would really help to know what the frame is. Without that information, all anyone here can do is guess.