As far as I know, CGCF is used to get the extra lower notes. As you have a 5 string, you already have 2 extra notes compared to CGCF in standard tuning! I'd recommend keeping the standard tuning or tune all strings down equally(eg from low to high ADGCF) so you still have same patterns on all strings. Makes thing easier.
Unless he needs that open C alot. This was my case, and he might consider tuning UP his b string to a C, or, for super heavy results, tune as he mentioned, dropping ALL strings 2 steps down.
Cytrex's seems to be the easiest method. drop the B to an A. Than you get the Octave from the Open A and a 2nd Fret on the G as you would originally with the B to 2nd fret D.
You could also just go with a 4 string bass. If funds allow.
Im in ecaxtly the same boat. My guitarist tunes CGCFwhatever whatever, and since i have a 5 string, i dont know what to put the B to. ive kept it to a C, but i need somethong else so i can play around more and explore the notes.
Should i make the B string an A or a G?
Is this what you asking Obsolex?
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Re-invented for the 5th time over.